refactor: simplify data connections from many-to-many site assignment to direct site ownership

Replace SiteDataConnectionAssignment join table with a direct SiteId FK on DataConnection,
simplifying the data model, repositories, UI, CLI, and deployment service.

src/ScadaLink.CLI/Commands/DataConnectionCommands.cs

@@ -15,8 +15,6 @@ public static class DataConnectionCommands
         command.Add(BuildCreate(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildUpdate(urlOption, formatOption, usernameOption, passwordOption));
         command.Add(BuildDelete(urlOption, formatOption, usernameOption, passwordOption));
-        command.Add(BuildAssign(urlOption, formatOption, usernameOption, passwordOption));
-        command.Add(BuildUnassign(urlOption, formatOption, usernameOption, passwordOption));
 
         return command;
     }
@@ -60,49 +58,41 @@ public static class DataConnectionCommands
         return cmd;
     }
 
-    private static Command BuildUnassign(Option<string> urlOption, Option<string> formatOption, Option<string> usernameOption, Option<string> passwordOption)
-    {
-        var idOption = new Option<int>("--assignment-id") { Description = "Assignment ID", Required = true };
-        var cmd = new Command("unassign") { Description = "Unassign a data connection from a site" };
-        cmd.Add(idOption);
-        cmd.SetAction(async (ParseResult result) =>
-        {
-            var id = result.GetValue(idOption);
-            return await CommandHelpers.ExecuteCommandAsync(
-                result, urlOption, formatOption, usernameOption, passwordOption, new UnassignDataConnectionFromSiteCommand(id));
-        });
-        return cmd;
-    }
-
     private static Command BuildList(Option<string> urlOption, Option<string> formatOption, Option<string> usernameOption, Option<string> passwordOption)
     {
-        var cmd = new Command("list") { Description = "List all data connections" };
+        var siteIdOption = new Option<int?>("--site-id") { Description = "Filter by site ID" };
+        var cmd = new Command("list") { Description = "List data connections" };
+        cmd.Add(siteIdOption);
         cmd.SetAction(async (ParseResult result) =>
         {
+            var siteId = result.GetValue(siteIdOption);
             return await CommandHelpers.ExecuteCommandAsync(
-                result, urlOption, formatOption, usernameOption, passwordOption, new ListDataConnectionsCommand());
+                result, urlOption, formatOption, usernameOption, passwordOption, new ListDataConnectionsCommand(siteId));
         });
         return cmd;
     }
 
     private static Command BuildCreate(Option<string> urlOption, Option<string> formatOption, Option<string> usernameOption, Option<string> passwordOption)
     {
+        var siteIdOption = new Option<int>("--site-id") { Description = "Site ID", Required = true };
         var nameOption = new Option<string>("--name") { Description = "Connection name", Required = true };
         var protocolOption = new Option<string>("--protocol") { Description = "Protocol (e.g. OpcUa)", Required = true };
         var configOption = new Option<string?>("--configuration") { Description = "Connection configuration JSON" };
 
         var cmd = new Command("create") { Description = "Create a new data connection" };
+        cmd.Add(siteIdOption);
         cmd.Add(nameOption);
         cmd.Add(protocolOption);
         cmd.Add(configOption);
         cmd.SetAction(async (ParseResult result) =>
         {
+            var siteId = result.GetValue(siteIdOption);
             var name = result.GetValue(nameOption)!;
             var protocol = result.GetValue(protocolOption)!;
             var config = result.GetValue(configOption);
             return await CommandHelpers.ExecuteCommandAsync(
                 result, urlOption, formatOption, usernameOption, passwordOption,
-                new CreateDataConnectionCommand(name, protocol, config));
+                new CreateDataConnectionCommand(siteId, name, protocol, config));
         });
         return cmd;
     }
@@ -120,23 +110,4 @@ public static class DataConnectionCommands
         });
         return cmd;
     }
-
-    private static Command BuildAssign(Option<string> urlOption, Option<string> formatOption, Option<string> usernameOption, Option<string> passwordOption)
-    {
-        var connectionIdOption = new Option<int>("--connection-id") { Description = "Data connection ID", Required = true };
-        var siteIdOption = new Option<int>("--site-id") { Description = "Site ID", Required = true };
-
-        var cmd = new Command("assign") { Description = "Assign a data connection to a site" };
-        cmd.Add(connectionIdOption);
-        cmd.Add(siteIdOption);
-        cmd.SetAction(async (ParseResult result) =>
-        {
-            var connectionId = result.GetValue(connectionIdOption);
-            var siteId = result.GetValue(siteIdOption);
-            return await CommandHelpers.ExecuteCommandAsync(
-                result, urlOption, formatOption, usernameOption, passwordOption,
-                new AssignDataConnectionToSiteCommand(connectionId, siteId));
-        });
-        return cmd;
-    }
 }

src/ScadaLink.CLI/README.md

@@ -603,22 +603,27 @@ scadalink --url <url> data-connection get --id <int>
 
 #### `data-connection list`
 
-List all configured data connections.
+List data connections, optionally filtered by site.
 
 ```sh
-scadalink --url <url> data-connection list
+scadalink --url <url> data-connection list [--site-id <int>]
-```
-
-#### `data-connection create`
-
-Create a new data connection definition.
-
-```sh
-scadalink --url <url> data-connection create --name <string> --protocol <string> [--configuration <json>]
 ```
 
 | Option | Required | Description |
 |--------|----------|-------------|
+| `--site-id` | no | Filter by site ID |
+
+#### `data-connection create`
+
+Create a new data connection belonging to a specific site.
+
+```sh
+scadalink --url <url> data-connection create --site-id <int> --name <string> --protocol <string> [--configuration <json>]
+```
+
+| Option | Required | Description |
+|--------|----------|-------------|
+| `--site-id` | yes | Site ID the connection belongs to |
 | `--name` | yes | Connection name |
 | `--protocol` | yes | Protocol identifier (e.g. `OpcUa`) |
 | `--configuration` | no | Protocol-specific configuration as a JSON string |
@@ -650,32 +655,6 @@ scadalink --url <url> data-connection delete --id <int>
 |--------|----------|-------------|
 | `--id` | yes | Data connection ID |
 
-#### `data-connection assign`
-
-Assign a data connection to a site.
-
-```sh
-scadalink --url <url> data-connection assign --connection-id <int> --site-id <int>
-```
-
-| Option | Required | Description |
-|--------|----------|-------------|
-| `--connection-id` | yes | Data connection ID |
-| `--site-id` | yes | Site ID |
-
-#### `data-connection unassign`
-
-Remove a data connection assignment from a site.
-
-```sh
-scadalink --url <url> data-connection unassign --connection-id <int> --site-id <int>
-```
-
-| Option | Required | Description |
-|--------|----------|-------------|
-| `--connection-id` | yes | Data connection ID |
-| `--site-id` | yes | Site ID |
-
 ---
 
 ### `external-system` — Manage external HTTP systems

src/ScadaLink.CentralUI/Components/Pages/Admin/DataConnectionForm.razor

@@ -21,6 +21,26 @@
     {
         <div class="card mb-3">
             <div class="card-body">
+                @if (Id.HasValue)
+                {
+                    <div class="mb-2">
+                        <label class="form-label small">Site</label>
+                        <input type="text" class="form-control form-control-sm" value="@(_siteName)" disabled />
+                    </div>
+                }
+                else
+                {
+                    <div class="mb-2">
+                        <label class="form-label small">Site</label>
+                        <select class="form-select form-select-sm" @bind="_formSiteId">
+                            <option value="0">Select site...</option>
+                            @foreach (var site in _sites)
+                            {
+                                <option value="@site.Id">@site.Name</option>
+                            }
+                        </select>
+                    </div>
+                }
                 <div class="mb-2">
                     <label class="form-label small">Name</label>
                     <input type="text" class="form-control form-control-sm" @bind="_formName" />
@@ -57,6 +77,9 @@
 
     private bool _loading = true;
     private DataConnection? _editingConnection;
+    private List<Site> _sites = new();
+    private int _formSiteId;
+    private string _siteName = string.Empty;
     private string _formName = string.Empty;
     private string _formProtocol = string.Empty;
     private string? _formConfiguration;
@@ -64,6 +87,8 @@
 
     protected override async Task OnInitializedAsync()
     {
+        _sites = (await SiteRepository.GetAllSitesAsync()).ToList();
+
         if (Id.HasValue)
         {
             try
@@ -71,6 +96,8 @@
                 _editingConnection = await SiteRepository.GetDataConnectionByIdAsync(Id.Value);
                 if (_editingConnection != null)
                 {
+                    _formSiteId = _editingConnection.SiteId;
+                    _siteName = _sites.FirstOrDefault(s => s.Id == _formSiteId)?.Name ?? $"Site {_formSiteId}";
                     _formName = _editingConnection.Name;
                     _formProtocol = _editingConnection.Protocol;
                     _formConfiguration = _editingConnection.Configuration;
@@ -87,6 +114,7 @@
     private async Task SaveConnection()
     {
         _formError = null;
+        if (_formSiteId == 0) { _formError = "Site is required."; return; }
         if (string.IsNullOrWhiteSpace(_formName)) { _formError = "Name is required."; return; }
         if (string.IsNullOrWhiteSpace(_formProtocol)) { _formError = "Protocol is required."; return; }
 
@@ -101,7 +129,7 @@
             }
             else
             {
-                var conn = new DataConnection(_formName.Trim(), _formProtocol)
+                var conn = new DataConnection(_formName.Trim(), _formProtocol, _formSiteId)
                 {
                     Configuration = _formConfiguration?.Trim()
                 };

src/ScadaLink.CentralUI/Components/Pages/Admin/DataConnections.razor

@@ -25,58 +25,14 @@
     }
     else
     {
-        @* Assignment form *@
-        @if (_showAssignForm)
-        {
-            <div class="card mb-3">
-                <div class="card-body">
-                    <h6 class="card-title">Assign Connection to Site</h6>
-                    <div class="row g-2 align-items-end">
-                        <div class="col-md-3">
-                            <label class="form-label small">Connection</label>
-                            <select class="form-select form-select-sm" @bind="_assignConnectionId">
-                                <option value="0">Select connection...</option>
-                                @foreach (var conn in _connections)
-                                {
-                                    <option value="@conn.Id">@conn.Name (@conn.Protocol)</option>
-                                }
-                            </select>
-                        </div>
-                        <div class="col-md-3">
-                            <label class="form-label small">Site</label>
-                            <select class="form-select form-select-sm" @bind="_assignSiteId">
-                                <option value="0">Select site...</option>
-                                @foreach (var site in _sites)
-                                {
-                                    <option value="@site.Id">@site.Name</option>
-                                }
-                            </select>
-                        </div>
-                        <div class="col-md-3">
-                            <button class="btn btn-success btn-sm me-1" @onclick="SaveAssignment">Assign</button>
-                            <button class="btn btn-outline-secondary btn-sm" @onclick="CancelAssignForm">Cancel</button>
-                        </div>
-                    </div>
-                    @if (_assignError != null)
-                    {
-                        <div class="text-danger small mt-1">@_assignError</div>
-                    }
-                </div>
-            </div>
-        }
-
-        <div class="mb-2">
-            <button class="btn btn-outline-info btn-sm" @onclick="ShowAssignForm">Assign to Site</button>
-        </div>
-
         <table class="table table-sm table-striped table-hover">
             <thead class="table-dark">
                 <tr>
                     <th>ID</th>
                     <th>Name</th>
                     <th>Protocol</th>
+                    <th>Site</th>
                     <th>Configuration</th>
-                    <th>Assigned Sites</th>
                     <th style="width: 160px;">Actions</th>
                 </tr>
             </thead>
@@ -93,29 +49,8 @@
                         <td>@conn.Id</td>
                         <td>@conn.Name</td>
                         <td><span class="badge bg-secondary">@conn.Protocol</span></td>
+                        <td>@(_siteLookup.GetValueOrDefault(conn.SiteId)?.Name ?? $"Site {conn.SiteId}")</td>
                         <td class="text-muted small text-truncate" style="max-width: 300px;">@(conn.Configuration ?? "—")</td>
-                        <td>
-                            @{
-                                var assignedSites = _connectionSites.GetValueOrDefault(conn.Id);
-                            }
-                            @if (assignedSites != null && assignedSites.Count > 0)
-                            {
-                                @foreach (var assignment in assignedSites)
-                                {
-                                    var siteName = _sites.FirstOrDefault(s => s.Id == assignment.SiteId)?.Name ?? $"Site {assignment.SiteId}";
-                                    <span class="badge bg-info text-dark me-1">
-                                        @siteName
-                                        <button type="button" class="btn-close btn-close-white ms-1"
-                                                style="font-size: 0.5rem;"
-                                                @onclick="() => RemoveAssignment(assignment)"></button>
-                                    </span>
-                                }
-                            }
-                            else
-                            {
-                                <span class="text-muted small">None</span>
-                            }
-                        </td>
                         <td>
                             <button class="btn btn-outline-primary btn-sm py-0 px-1 me-1"
                                     @onclick='() => NavigationManager.NavigateTo($"/admin/data-connections/{conn.Id}/edit")'>Edit</button>
@@ -131,16 +66,10 @@
 
 @code {
     private List<DataConnection> _connections = new();
-    private List<Site> _sites = new();
+    private Dictionary<int, Site> _siteLookup = new();
-    private Dictionary<int, List<SiteDataConnectionAssignment>> _connectionSites = new();
     private bool _loading = true;
     private string? _errorMessage;
 
-    private bool _showAssignForm;
-    private int _assignConnectionId;
-    private int _assignSiteId;
-    private string? _assignError;
-
     private ToastNotification _toast = default!;
     private ConfirmDialog _confirmDialog = default!;
 
@@ -155,24 +84,9 @@
         _errorMessage = null;
         try
         {
-            _sites = (await SiteRepository.GetAllSitesAsync()).ToList();
+            var sites = await SiteRepository.GetAllSitesAsync();
+            _siteLookup = sites.ToDictionary(s => s.Id);
             _connections = (await SiteRepository.GetAllDataConnectionsAsync()).ToList();
-
-            // Load site assignments for each connection
-            _connectionSites.Clear();
-            foreach (var site in _sites)
-            {
-                var siteConns = await SiteRepository.GetDataConnectionsBySiteIdAsync(site.Id);
-                foreach (var conn in siteConns)
-                {
-                    if (!_connectionSites.ContainsKey(conn.Id))
-                        _connectionSites[conn.Id] = new List<SiteDataConnectionAssignment>();
-
-                    var assignment = await SiteRepository.GetSiteDataConnectionAssignmentAsync(site.Id, conn.Id);
-                    if (assignment != null)
-                        _connectionSites[conn.Id].Add(assignment);
-                }
-            }
         }
         catch (Exception ex)
         {
@@ -199,58 +113,4 @@
             _toast.ShowError($"Delete failed: {ex.Message}");
         }
     }
-
-    private void ShowAssignForm()
-    {
-        _assignConnectionId = 0;
-        _assignSiteId = 0;
-        _assignError = null;
-        _showAssignForm = true;
-    }
-
-    private void CancelAssignForm()
-    {
-        _showAssignForm = false;
-        _assignError = null;
-    }
-
-    private async Task SaveAssignment()
-    {
-        _assignError = null;
-        if (_assignConnectionId == 0) { _assignError = "Select a connection."; return; }
-        if (_assignSiteId == 0) { _assignError = "Select a site."; return; }
-
-        try
-        {
-            var assignment = new SiteDataConnectionAssignment
-            {
-                SiteId = _assignSiteId,
-                DataConnectionId = _assignConnectionId
-            };
-            await SiteRepository.AddSiteDataConnectionAssignmentAsync(assignment);
-            await SiteRepository.SaveChangesAsync();
-            _showAssignForm = false;
-            _toast.ShowSuccess("Connection assigned to site.");
-            await LoadDataAsync();
-        }
-        catch (Exception ex)
-        {
-            _assignError = $"Assignment failed: {ex.Message}";
-        }
-    }
-
-    private async Task RemoveAssignment(SiteDataConnectionAssignment assignment)
-    {
-        try
-        {
-            await SiteRepository.DeleteSiteDataConnectionAssignmentAsync(assignment.Id);
-            await SiteRepository.SaveChangesAsync();
-            _toast.ShowSuccess("Assignment removed.");
-            await LoadDataAsync();
-        }
-        catch (Exception ex)
-        {
-            _toast.ShowError($"Remove failed: {ex.Message}");
-        }
-    }
 }

src/ScadaLink.CentralUI/Components/Pages/Admin/Sites.razor

@@ -178,10 +178,9 @@
         _deploying = true;
         try
         {
-            var command = await ArtifactDeploymentService.BuildDeployArtifactsCommandAsync();
             var user = await GetCurrentUserAsync();
             var result = await ArtifactDeploymentService.RetryForSiteAsync(
-                site.SiteIdentifier, command, user);
+                site.Id, site.SiteIdentifier, user);
 
             if (result.IsSuccess)
                 _toast.ShowSuccess($"Artifacts deployed to '{site.Name}'.");
@@ -203,9 +202,8 @@
         _deploying = true;
         try
         {
-            var command = await ArtifactDeploymentService.BuildDeployArtifactsCommandAsync();
             var user = await GetCurrentUserAsync();
-            var result = await ArtifactDeploymentService.DeployToAllSitesAsync(command, user);
+            var result = await ArtifactDeploymentService.DeployToAllSitesAsync(user);
 
             if (result.IsSuccess)
             {

src/ScadaLink.CentralUI/Components/Pages/Deployment/Instances.razor

@@ -697,13 +697,8 @@
         var attrs = await TemplateEngineRepository.GetAttributesByTemplateIdAsync(inst.TemplateId);
         _bindingDataSourceAttrs = attrs.Where(a => !string.IsNullOrEmpty(a.DataSourceReference)).ToList();
 
-        // Load data connections for this site
+        // Load data connections for this site (each connection now belongs to exactly one site)
         _siteConnections = (await SiteRepository.GetDataConnectionsBySiteIdAsync(inst.SiteId)).ToList();
-        if (_siteConnections.Count == 0)
-        {
-            // Also show unassigned connections (they may not be assigned to a site yet)
-            _siteConnections = (await SiteRepository.GetAllDataConnectionsAsync()).ToList();
-        }
 
         // Load existing bindings
         var existingBindings = await TemplateEngineRepository.GetBindingsByInstanceIdAsync(inst.Id);

src/ScadaLink.Commons/Entities/Sites/DataConnection.cs

@@ -3,13 +3,15 @@ namespace ScadaLink.Commons.Entities.Sites;
 public class DataConnection
 {
     public int Id { get; set; }
+    public int SiteId { get; set; }
     public string Name { get; set; }
     public string Protocol { get; set; }
     public string? Configuration { get; set; }
 
-    public DataConnection(string name, string protocol)
+    public DataConnection(string name, string protocol, int siteId)
     {
         Name = name ?? throw new ArgumentNullException(nameof(name));
         Protocol = protocol ?? throw new ArgumentNullException(nameof(protocol));
+        SiteId = siteId;
     }
 }

src/ScadaLink.Commons/Entities/Sites/SiteDataConnectionAssignment.cs

@@ -1,8 +0,0 @@
-namespace ScadaLink.Commons.Entities.Sites;
-
-public class SiteDataConnectionAssignment
-{
-    public int Id { get; set; }
-    public int SiteId { get; set; }
-    public int DataConnectionId { get; set; }
-}

src/ScadaLink.Commons/Interfaces/Repositories/ICentralUiRepository.cs

@@ -10,7 +10,7 @@ public interface ICentralUiRepository
 {
     Task<IReadOnlyList<Site>> GetAllSitesAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<DataConnection>> GetDataConnectionsBySiteIdAsync(int siteId, CancellationToken cancellationToken = default);
-    Task<IReadOnlyList<SiteDataConnectionAssignment>> GetAllSiteDataConnectionAssignmentsAsync(CancellationToken cancellationToken = default);
+    Task<IReadOnlyList<DataConnection>> GetAllDataConnectionsAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<Template>> GetTemplateTreeAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<Instance>> GetInstancesFilteredAsync(int? siteId = null, int? templateId = null, string? searchTerm = null, CancellationToken cancellationToken = default);
     Task<IReadOnlyList<DeploymentRecord>> GetRecentDeploymentsAsync(int count, CancellationToken cancellationToken = default);

src/ScadaLink.Commons/Interfaces/Repositories/ISiteRepository.cs

@@ -24,11 +24,6 @@ public interface ISiteRepository
     Task UpdateDataConnectionAsync(DataConnection connection, CancellationToken cancellationToken = default);
     Task DeleteDataConnectionAsync(int id, CancellationToken cancellationToken = default);
 
-    // Site-Connection Assignments
-    Task<SiteDataConnectionAssignment?> GetSiteDataConnectionAssignmentAsync(int siteId, int dataConnectionId, CancellationToken cancellationToken = default);
-    Task AddSiteDataConnectionAssignmentAsync(SiteDataConnectionAssignment assignment, CancellationToken cancellationToken = default);
-    Task DeleteSiteDataConnectionAssignmentAsync(int id, CancellationToken cancellationToken = default);
-
     // Instances (for deletion constraint checks)
     Task<IReadOnlyList<Instance>> GetInstancesBySiteIdAsync(int siteId, CancellationToken cancellationToken = default);
 

src/ScadaLink.Commons/Messages/Management/DataConnectionCommands.cs

@@ -1,9 +1,7 @@
 namespace ScadaLink.Commons.Messages.Management;
 
-public record ListDataConnectionsCommand;
+public record ListDataConnectionsCommand(int? SiteId = null);
 public record GetDataConnectionCommand(int DataConnectionId);
-public record CreateDataConnectionCommand(string Name, string Protocol, string? Configuration);
+public record CreateDataConnectionCommand(int SiteId, string Name, string Protocol, string? Configuration);
 public record UpdateDataConnectionCommand(int DataConnectionId, string Name, string Protocol, string? Configuration);
 public record DeleteDataConnectionCommand(int DataConnectionId);
-public record AssignDataConnectionToSiteCommand(int DataConnectionId, int SiteId);
-public record UnassignDataConnectionFromSiteCommand(int AssignmentId);

src/ScadaLink.ConfigurationDatabase/Configurations/SiteConfiguration.cs

@@ -46,26 +46,11 @@ public class DataConnectionConfiguration : IEntityTypeConfiguration<DataConnecti
         builder.Property(d => d.Configuration)
             .HasMaxLength(4000);
 
-        builder.HasIndex(d => d.Name).IsUnique();
-    }
-}
-
-public class SiteDataConnectionAssignmentConfiguration : IEntityTypeConfiguration<SiteDataConnectionAssignment>
-{
-    public void Configure(EntityTypeBuilder<SiteDataConnectionAssignment> builder)
-    {
-        builder.HasKey(a => a.Id);
-
         builder.HasOne<Site>()
             .WithMany()
-            .HasForeignKey(a => a.SiteId)
+            .HasForeignKey(d => d.SiteId)
-            .OnDelete(DeleteBehavior.Cascade);
+            .OnDelete(DeleteBehavior.Restrict);
 
-        builder.HasOne<DataConnection>()
+        builder.HasIndex(d => new { d.SiteId, d.Name }).IsUnique();
-            .WithMany()
-            .HasForeignKey(a => a.DataConnectionId)
-            .OnDelete(DeleteBehavior.Cascade);
-
-        builder.HasIndex(a => new { a.SiteId, a.DataConnectionId }).IsUnique();
     }
 }

src/ScadaLink.ConfigurationDatabase/Migrations/20260321210402_AddSiteIdToDataConnections.cs

@@ -0,0 +1,184 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ScadaLink.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddSiteIdToDataConnections : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // Step 1: Drop old unique index on Name (allows duplicate names across sites)
+            migrationBuilder.DropIndex(
+                name: "IX_DataConnections_Name",
+                table: "DataConnections");
+
+            // Step 2: Add nullable SiteId column
+            migrationBuilder.AddColumn<int>(
+                name: "SiteId",
+                table: "DataConnections",
+                type: "int",
+                nullable: true);
+
+            // Step 3: Migrate data from SiteDataConnectionAssignments
+            migrationBuilder.Sql(@"
+                -- Phase A: Assign the first site to each existing DataConnection
+                UPDATE dc
+                SET dc.SiteId = a.SiteId
+                FROM DataConnections dc
+                INNER JOIN (
+                    SELECT DataConnectionId, MIN(SiteId) AS SiteId
+                    FROM SiteDataConnectionAssignments
+                    GROUP BY DataConnectionId
+                ) a ON dc.Id = a.DataConnectionId
+                WHERE dc.SiteId IS NULL;
+
+                -- Phase B: For connections assigned to additional sites, create copies
+                -- and update InstanceConnectionBindings to point to the new copy
+                DECLARE @AssignSiteId INT, @AssignConnId INT, @NewConnId INT;
+                DECLARE @OrigName NVARCHAR(200), @OrigProtocol NVARCHAR(50), @OrigConfig NVARCHAR(4000);
+
+                DECLARE assignment_cursor CURSOR FOR
+                    SELECT a.SiteId, a.DataConnectionId
+                    FROM SiteDataConnectionAssignments a
+                    INNER JOIN DataConnections dc ON a.DataConnectionId = dc.Id
+                    WHERE dc.SiteId <> a.SiteId;
+
+                OPEN assignment_cursor;
+                FETCH NEXT FROM assignment_cursor INTO @AssignSiteId, @AssignConnId;
+
+                WHILE @@FETCH_STATUS = 0
+                BEGIN
+                    SELECT @OrigName = Name, @OrigProtocol = Protocol, @OrigConfig = Configuration
+                    FROM DataConnections WHERE Id = @AssignConnId;
+
+                    INSERT INTO DataConnections (SiteId, Name, Protocol, Configuration)
+                    VALUES (@AssignSiteId, @OrigName, @OrigProtocol, @OrigConfig);
+
+                    SET @NewConnId = SCOPE_IDENTITY();
+
+                    -- Update bindings for instances on this site to point to the new connection
+                    UPDATE icb
+                    SET icb.DataConnectionId = @NewConnId
+                    FROM InstanceConnectionBindings icb
+                    INNER JOIN Instances i ON icb.InstanceId = i.Id
+                    WHERE icb.DataConnectionId = @AssignConnId
+                      AND i.SiteId = @AssignSiteId;
+
+                    FETCH NEXT FROM assignment_cursor INTO @AssignSiteId, @AssignConnId;
+                END
+
+                CLOSE assignment_cursor;
+                DEALLOCATE assignment_cursor;
+
+                -- Phase C: Handle any DataConnections not assigned to any site
+                -- (assign to the first site as a fallback)
+                UPDATE dc
+                SET dc.SiteId = (SELECT TOP 1 Id FROM Sites ORDER BY Id)
+                FROM DataConnections dc
+                WHERE dc.SiteId IS NULL;
+            ");
+
+            // Step 4: Make SiteId non-nullable
+            migrationBuilder.AlterColumn<int>(
+                name: "SiteId",
+                table: "DataConnections",
+                type: "int",
+                nullable: false,
+                defaultValue: 0,
+                oldClrType: typeof(int),
+                oldType: "int",
+                oldNullable: true);
+
+            // Step 5: Add composite unique index and FK
+            migrationBuilder.CreateIndex(
+                name: "IX_DataConnections_SiteId_Name",
+                table: "DataConnections",
+                columns: new[] { "SiteId", "Name" },
+                unique: true);
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_DataConnections_Sites_SiteId",
+                table: "DataConnections",
+                column: "SiteId",
+                principalTable: "Sites",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Restrict);
+
+            // Step 6: Drop SiteDataConnectionAssignments table
+            migrationBuilder.DropTable(
+                name: "SiteDataConnectionAssignments");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            // Recreate SiteDataConnectionAssignments table
+            migrationBuilder.CreateTable(
+                name: "SiteDataConnectionAssignments",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    DataConnectionId = table.Column<int>(type: "int", nullable: false),
+                    SiteId = table.Column<int>(type: "int", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_SiteDataConnectionAssignments", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_SiteDataConnectionAssignments_DataConnections_DataConnectionId",
+                        column: x => x.DataConnectionId,
+                        principalTable: "DataConnections",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                    table.ForeignKey(
+                        name: "FK_SiteDataConnectionAssignments_Sites_SiteId",
+                        column: x => x.SiteId,
+                        principalTable: "Sites",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_SiteDataConnectionAssignments_DataConnectionId",
+                table: "SiteDataConnectionAssignments",
+                column: "DataConnectionId");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_SiteDataConnectionAssignments_SiteId_DataConnectionId",
+                table: "SiteDataConnectionAssignments",
+                columns: new[] { "SiteId", "DataConnectionId" },
+                unique: true);
+
+            // Migrate data back
+            migrationBuilder.Sql(@"
+                INSERT INTO SiteDataConnectionAssignments (SiteId, DataConnectionId)
+                SELECT SiteId, Id FROM DataConnections;
+            ");
+
+            // Remove FK and composite index
+            migrationBuilder.DropForeignKey(
+                name: "FK_DataConnections_Sites_SiteId",
+                table: "DataConnections");
+
+            migrationBuilder.DropIndex(
+                name: "IX_DataConnections_SiteId_Name",
+                table: "DataConnections");
+
+            // Restore unique index on Name
+            migrationBuilder.CreateIndex(
+                name: "IX_DataConnections_Name",
+                table: "DataConnections",
+                column: "Name",
+                unique: true);
+
+            // Drop SiteId column
+            migrationBuilder.DropColumn(
+                name: "SiteId",
+                table: "DataConnections");
+        }
+    }
+}

src/ScadaLink.ConfigurationDatabase/Migrations/ScadaLinkDbContextModelSnapshot.cs

@@ -766,9 +766,12 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .HasMaxLength(50)
                         .HasColumnType("nvarchar(50)");
 
+                    b.Property<int>("SiteId")
+                        .HasColumnType("int");
+
                     b.HasKey("Id");
 
-                    b.HasIndex("Name")
+                    b.HasIndex("SiteId", "Name")
                         .IsUnique();
 
                     b.ToTable("DataConnections");
@@ -821,30 +824,6 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                     b.ToTable("Sites");
                 });
 
-            modelBuilder.Entity("ScadaLink.Commons.Entities.Sites.SiteDataConnectionAssignment", b =>
-                {
-                    b.Property<int>("Id")
-                        .ValueGeneratedOnAdd()
-                        .HasColumnType("int");
-
-                    SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
-
-                    b.Property<int>("DataConnectionId")
-                        .HasColumnType("int");
-
-                    b.Property<int>("SiteId")
-                        .HasColumnType("int");
-
-                    b.HasKey("Id");
-
-                    b.HasIndex("DataConnectionId");
-
-                    b.HasIndex("SiteId", "DataConnectionId")
-                        .IsUnique();
-
-                    b.ToTable("SiteDataConnectionAssignments");
-                });
-
             modelBuilder.Entity("ScadaLink.Commons.Entities.Templates.Template", b =>
                 {
                     b.Property<int>("Id")
@@ -1153,18 +1132,12 @@ namespace ScadaLink.ConfigurationDatabase.Migrations
                         .IsRequired();
                 });
 
-            modelBuilder.Entity("ScadaLink.Commons.Entities.Sites.SiteDataConnectionAssignment", b =>
+            modelBuilder.Entity("ScadaLink.Commons.Entities.Sites.DataConnection", b =>
                 {
-                    b.HasOne("ScadaLink.Commons.Entities.Sites.DataConnection", null)
-                        .WithMany()
-                        .HasForeignKey("DataConnectionId")
-                        .OnDelete(DeleteBehavior.Cascade)
-                        .IsRequired();
-
                     b.HasOne("ScadaLink.Commons.Entities.Sites.Site", null)
                         .WithMany()
                         .HasForeignKey("SiteId")
-                        .OnDelete(DeleteBehavior.Cascade)
+                        .OnDelete(DeleteBehavior.Restrict)
                         .IsRequired();
                 });
 

src/ScadaLink.ConfigurationDatabase/Repositories/CentralUiRepository.cs

@@ -27,17 +27,18 @@ public class CentralUiRepository : ICentralUiRepository
 
     public async Task<IReadOnlyList<DataConnection>> GetDataConnectionsBySiteIdAsync(int siteId, CancellationToken cancellationToken = default)
     {
-        return await _context.SiteDataConnectionAssignments
+        return await _context.DataConnections
             .AsNoTracking()
-            .Where(a => a.SiteId == siteId)
+            .Where(d => d.SiteId == siteId)
-            .Join(_context.DataConnections, a => a.DataConnectionId, d => d.Id, (_, d) => d)
+            .OrderBy(d => d.Name)
             .ToListAsync(cancellationToken);
     }
 
-    public async Task<IReadOnlyList<SiteDataConnectionAssignment>> GetAllSiteDataConnectionAssignmentsAsync(CancellationToken cancellationToken = default)
+    public async Task<IReadOnlyList<DataConnection>> GetAllDataConnectionsAsync(CancellationToken cancellationToken = default)
     {
-        return await _context.SiteDataConnectionAssignments
+        return await _context.DataConnections
             .AsNoTracking()
+            .OrderBy(d => d.Name)
             .ToListAsync(cancellationToken);
     }
 

src/ScadaLink.ConfigurationDatabase/Repositories/SiteRepository.cs

@@ -76,13 +76,8 @@ public class SiteRepository : ISiteRepository
 
     public async Task<IReadOnlyList<DataConnection>> GetDataConnectionsBySiteIdAsync(int siteId, CancellationToken cancellationToken = default)
     {
-        var connectionIds = await _dbContext.SiteDataConnectionAssignments
-            .Where(a => a.SiteId == siteId)
-            .Select(a => a.DataConnectionId)
-            .ToListAsync(cancellationToken);
-
         return await _dbContext.DataConnections
-            .Where(c => connectionIds.Contains(c.Id))
+            .Where(c => c.SiteId == siteId)
             .OrderBy(c => c.Name)
             .ToListAsync(cancellationToken);
     }
@@ -107,43 +102,13 @@ public class SiteRepository : ISiteRepository
         }
         else
         {
-            var stub = new DataConnection("stub", "stub") { Id = id };
+            var stub = new DataConnection("stub", "stub", 0) { Id = id };
             _dbContext.DataConnections.Attach(stub);
             _dbContext.DataConnections.Remove(stub);
         }
         return Task.CompletedTask;
     }
 
-    // --- Site-Connection Assignments ---
-
-    public async Task<SiteDataConnectionAssignment?> GetSiteDataConnectionAssignmentAsync(
-        int siteId, int dataConnectionId, CancellationToken cancellationToken = default)
-    {
-        return await _dbContext.SiteDataConnectionAssignments
-            .FirstOrDefaultAsync(a => a.SiteId == siteId && a.DataConnectionId == dataConnectionId, cancellationToken);
-    }
-
-    public async Task AddSiteDataConnectionAssignmentAsync(SiteDataConnectionAssignment assignment, CancellationToken cancellationToken = default)
-    {
-        await _dbContext.SiteDataConnectionAssignments.AddAsync(assignment, cancellationToken);
-    }
-
-    public Task DeleteSiteDataConnectionAssignmentAsync(int id, CancellationToken cancellationToken = default)
-    {
-        var entity = _dbContext.SiteDataConnectionAssignments.Local.FirstOrDefault(a => a.Id == id);
-        if (entity != null)
-        {
-            _dbContext.SiteDataConnectionAssignments.Remove(entity);
-        }
-        else
-        {
-            var stub = new SiteDataConnectionAssignment { Id = id };
-            _dbContext.SiteDataConnectionAssignments.Attach(stub);
-            _dbContext.SiteDataConnectionAssignments.Remove(stub);
-        }
-        return Task.CompletedTask;
-    }
-
     // --- Instances (for deletion constraint checks) ---
 
     public async Task<IReadOnlyList<Instance>> GetInstancesBySiteIdAsync(int siteId, CancellationToken cancellationToken = default)

src/ScadaLink.ConfigurationDatabase/ScadaLinkDbContext.cs

@@ -35,7 +35,6 @@ public class ScadaLinkDbContext : DbContext, IDataProtectionKeyContext
     // Sites
     public DbSet<Site> Sites => Set<Site>();
     public DbSet<DataConnection> DataConnections => Set<DataConnection>();
-    public DbSet<SiteDataConnectionAssignment> SiteDataConnectionAssignments => Set<SiteDataConnectionAssignment>();
 
     // Deployment
     public DbSet<DeploymentRecord> DeploymentRecords => Set<DeploymentRecord>();

src/ScadaLink.DeploymentManager/ArtifactDeploymentService.cs

@@ -55,16 +55,18 @@ public class ArtifactDeploymentService
     }
 
     /// <summary>
-    /// Collects all artifact types from repositories and builds a <see cref="DeployArtifactsCommand"/>.
+    /// Collects all artifact types from repositories and builds a <see cref="DeployArtifactsCommand"/>
+    /// scoped to a specific site's data connections.
     /// </summary>
     public async Task<DeployArtifactsCommand> BuildDeployArtifactsCommandAsync(
+        int siteId,
         CancellationToken cancellationToken = default)
     {
         var sharedScripts = await _templateRepo.GetAllSharedScriptsAsync(cancellationToken);
         var externalSystems = await _externalSystemRepo.GetAllExternalSystemsAsync(cancellationToken);
         var dbConnections = await _externalSystemRepo.GetAllDatabaseConnectionsAsync(cancellationToken);
         var notificationLists = await _notificationRepo.GetAllNotificationListsAsync(cancellationToken);
-        var dataConnections = await _siteRepo.GetAllDataConnectionsAsync(cancellationToken);
+        var dataConnections = await _siteRepo.GetDataConnectionsBySiteIdAsync(siteId, cancellationToken);
         var smtpConfigurations = await _notificationRepo.GetAllSmtpConfigurationsAsync(cancellationToken);
 
         // Map shared scripts
@@ -120,10 +122,10 @@ public class ArtifactDeploymentService
     }
 
     /// <summary>
-    /// Deploys artifacts to all sites. Returns per-site result matrix.
+    /// Deploys artifacts to all sites. Builds a per-site command with that site's data connections.
+    /// Returns per-site result matrix.
     /// </summary>
     public async Task<Result<ArtifactDeploymentSummary>> DeployToAllSitesAsync(
-        DeployArtifactsCommand command,
         string user,
         CancellationToken cancellationToken = default)
     {
@@ -131,9 +133,17 @@ public class ArtifactDeploymentService
         if (sites.Count == 0)
             return Result<ArtifactDeploymentSummary>.Failure("No sites configured.");
 
+        var deploymentId = Guid.NewGuid().ToString("N");
         var perSiteResults = new Dictionary<string, SiteArtifactResult>();
 
-        // Deploy to each site with per-site timeout
+        // Build per-site commands sequentially (DbContext is not thread-safe)
+        var siteCommands = new Dictionary<int, DeployArtifactsCommand>();
+        foreach (var site in sites)
+        {
+            siteCommands[site.Id] = await BuildDeployArtifactsCommandAsync(site.Id, cancellationToken);
+        }
+
+        // Deploy to each site in parallel with per-site timeout
         var tasks = sites.Select(async site =>
         {
             try
@@ -141,6 +151,8 @@ public class ArtifactDeploymentService
                 using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
                 cts.CancelAfter(_options.ArtifactDeploymentTimeoutPerSite);
 
+                var command = siteCommands[site.Id];
+
                 _logger.LogInformation(
                     "Deploying artifacts to site {SiteId} ({SiteName}), deploymentId={DeploymentId}",
                     site.SiteIdentifier, site.Name, command.DeploymentId);
@@ -188,13 +200,13 @@ public class ArtifactDeploymentService
         await _deploymentRepo.SaveChangesAsync(cancellationToken);
 
         var summary = new ArtifactDeploymentSummary(
-            command.DeploymentId,
+            deploymentId,
             results.ToList(),
             results.Count(r => r.Success),
             results.Count(r => !r.Success));
 
         await _auditService.LogAsync(user, "DeployArtifacts", "SystemArtifact",
-            command.DeploymentId, "Artifacts",
+            deploymentId, "Artifacts",
             new { summary.SuccessCount, summary.FailureCount },
             cancellationToken);
 
@@ -205,8 +217,8 @@ public class ArtifactDeploymentService
     /// WP-7: Retry artifact deployment to a specific site that previously failed.
     /// </summary>
     public async Task<Result<SiteArtifactResult>> RetryForSiteAsync(
-        string siteId,
+        int siteDbId,
-        DeployArtifactsCommand command,
+        string siteIdentifier,
         string user,
         CancellationToken cancellationToken = default)
     {
@@ -215,12 +227,13 @@ public class ArtifactDeploymentService
             using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
             cts.CancelAfter(_options.ArtifactDeploymentTimeoutPerSite);
 
-            var response = await _communicationService.DeployArtifactsAsync(siteId, command, cts.Token);
+            var command = await BuildDeployArtifactsCommandAsync(siteDbId, cts.Token);
+            var response = await _communicationService.DeployArtifactsAsync(siteIdentifier, command, cts.Token);
 
-            var result = new SiteArtifactResult(siteId, siteId, response.Success, response.ErrorMessage);
+            var result = new SiteArtifactResult(siteIdentifier, siteIdentifier, response.Success, response.ErrorMessage);
 
             await _auditService.LogAsync(user, "RetryArtifactDeployment", "SystemArtifact",
-                command.DeploymentId, siteId, new { response.Success }, cancellationToken);
+                command.DeploymentId, siteIdentifier, new { response.Success }, cancellationToken);
 
             return response.Success
                 ? Result<SiteArtifactResult>.Success(result)
@@ -228,7 +241,7 @@ public class ArtifactDeploymentService
         }
         catch (Exception ex)
         {
-            return Result<SiteArtifactResult>.Failure($"Retry failed for site {siteId}: {ex.Message}");
+            return Result<SiteArtifactResult>.Failure($"Retry failed for site {siteIdentifier}: {ex.Message}");
         }
     }
 }

src/ScadaLink.ManagementService/ManagementActor.cs

@@ -103,8 +103,6 @@ public class ManagementActor : ReceiveActor
             or UpdateSmtpConfigCommand
             or CreateDataConnectionCommand or UpdateDataConnectionCommand
             or DeleteDataConnectionCommand
-            or AssignDataConnectionToSiteCommand
-            or UnassignDataConnectionFromSiteCommand
             or AddTemplateAttributeCommand or UpdateTemplateAttributeCommand or DeleteTemplateAttributeCommand
             or AddTemplateAlarmCommand or UpdateTemplateAlarmCommand or DeleteTemplateAlarmCommand
             or AddTemplateScriptCommand or UpdateTemplateScriptCommand or DeleteTemplateScriptCommand
@@ -176,13 +174,11 @@ public class ManagementActor : ReceiveActor
             UpdateAreaCommand cmd => await HandleUpdateArea(sp, cmd, user.Username),
 
             // Data Connections
-            ListDataConnectionsCommand => await HandleListDataConnections(sp),
+            ListDataConnectionsCommand cmd => await HandleListDataConnections(sp, cmd),
             GetDataConnectionCommand cmd => await HandleGetDataConnection(sp, cmd),
             CreateDataConnectionCommand cmd => await HandleCreateDataConnection(sp, cmd, user.Username),
             UpdateDataConnectionCommand cmd => await HandleUpdateDataConnection(sp, cmd, user.Username),
             DeleteDataConnectionCommand cmd => await HandleDeleteDataConnection(sp, cmd, user.Username),
-            AssignDataConnectionToSiteCommand cmd => await HandleAssignDataConnectionToSite(sp, cmd, user.Username),
-            UnassignDataConnectionFromSiteCommand cmd => await HandleUnassignDataConnectionFromSite(sp, cmd, user.Username),
 
             // External Systems
             ListExternalSystemsCommand => await HandleListExternalSystems(sp),
@@ -676,9 +672,11 @@ public class ManagementActor : ReceiveActor
     // Data Connection handlers
     // ========================================================================
 
-    private static async Task<object?> HandleListDataConnections(IServiceProvider sp)
+    private static async Task<object?> HandleListDataConnections(IServiceProvider sp, ListDataConnectionsCommand cmd)
     {
         var repo = sp.GetRequiredService<ISiteRepository>();
+        if (cmd.SiteId.HasValue)
+            return await repo.GetDataConnectionsBySiteIdAsync(cmd.SiteId.Value);
         return await repo.GetAllDataConnectionsAsync();
     }
 
@@ -691,7 +689,7 @@ public class ManagementActor : ReceiveActor
     private static async Task<object?> HandleCreateDataConnection(IServiceProvider sp, CreateDataConnectionCommand cmd, string user)
     {
         var repo = sp.GetRequiredService<ISiteRepository>();
-        var conn = new DataConnection(cmd.Name, cmd.Protocol) { Configuration = cmd.Configuration };
+        var conn = new DataConnection(cmd.Name, cmd.Protocol, cmd.SiteId) { Configuration = cmd.Configuration };
         await repo.AddDataConnectionAsync(conn);
         await repo.SaveChangesAsync();
         await AuditAsync(sp, user, "Create", "DataConnection", conn.Id.ToString(), conn.Name, conn);
@@ -721,28 +719,6 @@ public class ManagementActor : ReceiveActor
         return true;
     }
 
-    private static async Task<object?> HandleAssignDataConnectionToSite(IServiceProvider sp, AssignDataConnectionToSiteCommand cmd, string user)
-    {
-        var repo = sp.GetRequiredService<ISiteRepository>();
-        var assignment = new SiteDataConnectionAssignment
-        {
-            SiteId = cmd.SiteId,
-            DataConnectionId = cmd.DataConnectionId
-        };
-        await repo.AddSiteDataConnectionAssignmentAsync(assignment);
-        await repo.SaveChangesAsync();
-        await AuditAsync(sp, user, "Assign", "DataConnection", cmd.DataConnectionId.ToString(), $"Site:{cmd.SiteId}", assignment);
-        return assignment;
-    }
-
-    private static async Task<object?> HandleUnassignDataConnectionFromSite(IServiceProvider sp, UnassignDataConnectionFromSiteCommand cmd, string user)
-    {
-        var repo = sp.GetRequiredService<ISiteRepository>();
-        await repo.DeleteSiteDataConnectionAssignmentAsync(cmd.AssignmentId);
-        await repo.SaveChangesAsync();
-        await AuditAsync(sp, user, "Unassign", "DataConnection", cmd.AssignmentId.ToString(), cmd.AssignmentId.ToString(), null);
-        return true;
-    }
 
     // ========================================================================
     // External System handlers
@@ -1011,8 +987,7 @@ public class ManagementActor : ReceiveActor
     private static async Task<object?> HandleDeployArtifacts(IServiceProvider sp, MgmtDeployArtifactsCommand cmd, string user)
     {
         var svc = sp.GetRequiredService<ArtifactDeploymentService>();
-        var command = await svc.BuildDeployArtifactsCommandAsync();
+        var result = await svc.DeployToAllSitesAsync(user);
-        var result = await svc.DeployToAllSitesAsync(command, user);
         return result.IsSuccess
             ? result.Value
             : throw new InvalidOperationException(result.Error);

src/ScadaLink.TemplateEngine/Services/SiteService.cs

@@ -8,9 +8,7 @@ namespace ScadaLink.TemplateEngine.Services;
 /// <summary>
 /// Site and data connection management.
 /// - Site CRUD (name, identifier, description)
-/// - Data connection CRUD (name, protocol, config)
+/// - Data connection CRUD (name, protocol, config) — each connection belongs to exactly one site
-/// - Assign connections to sites
-/// - Connection names not standardized across sites
 /// - Audit logging
 /// </summary>
 public class SiteService
@@ -98,7 +96,7 @@ public class SiteService
     // --- Data Connection CRUD ---
 
     public async Task<Result<DataConnection>> CreateDataConnectionAsync(
-        string name, string protocol, string? configuration, string user,
+        int siteId, string name, string protocol, string? configuration, string user,
         CancellationToken cancellationToken = default)
     {
         if (string.IsNullOrWhiteSpace(name))
@@ -106,7 +104,7 @@ public class SiteService
         if (string.IsNullOrWhiteSpace(protocol))
             return Result<DataConnection>.Failure("Protocol is required.");
 
-        var connection = new DataConnection(name, protocol) { Configuration = configuration };
+        var connection = new DataConnection(name, protocol, siteId) { Configuration = configuration };
         await _repository.AddDataConnectionAsync(connection, cancellationToken);
         await _repository.SaveChangesAsync(cancellationToken);
 
@@ -151,56 +149,4 @@ public class SiteService
 
         return Result<bool>.Success(true);
     }
-
-    // --- Site-Connection Assignment ---
-
-    public async Task<Result<SiteDataConnectionAssignment>> AssignConnectionToSiteAsync(
-        int siteId, int dataConnectionId, string user,
-        CancellationToken cancellationToken = default)
-    {
-        var site = await _repository.GetSiteByIdAsync(siteId, cancellationToken);
-        if (site == null)
-            return Result<SiteDataConnectionAssignment>.Failure($"Site with ID {siteId} not found.");
-
-        var connection = await _repository.GetDataConnectionByIdAsync(dataConnectionId, cancellationToken);
-        if (connection == null)
-            return Result<SiteDataConnectionAssignment>.Failure($"Data connection with ID {dataConnectionId} not found.");
-
-        // Check if assignment already exists
-        var existing = await _repository.GetSiteDataConnectionAssignmentAsync(siteId, dataConnectionId, cancellationToken);
-        if (existing != null)
-            return Result<SiteDataConnectionAssignment>.Failure(
-                $"Data connection '{connection.Name}' is already assigned to site '{site.Name}'.");
-
-        var assignment = new SiteDataConnectionAssignment
-        {
-            SiteId = siteId,
-            DataConnectionId = dataConnectionId
-        };
-
-        await _repository.AddSiteDataConnectionAssignmentAsync(assignment, cancellationToken);
-        await _repository.SaveChangesAsync(cancellationToken);
-
-        await _auditService.LogAsync(user, "AssignConnection", "SiteDataConnectionAssignment",
-            assignment.Id.ToString(), $"{site.Name}/{connection.Name}", assignment, cancellationToken);
-
-        return Result<SiteDataConnectionAssignment>.Success(assignment);
-    }
-
-    public async Task<Result<bool>> RemoveConnectionFromSiteAsync(
-        int siteId, int dataConnectionId, string user,
-        CancellationToken cancellationToken = default)
-    {
-        var assignment = await _repository.GetSiteDataConnectionAssignmentAsync(siteId, dataConnectionId, cancellationToken);
-        if (assignment == null)
-            return Result<bool>.Failure("Assignment not found.");
-
-        await _repository.DeleteSiteDataConnectionAssignmentAsync(assignment.Id, cancellationToken);
-        await _repository.SaveChangesAsync(cancellationToken);
-
-        await _auditService.LogAsync(user, "RemoveConnection", "SiteDataConnectionAssignment",
-            assignment.Id.ToString(), $"Site:{siteId}/Conn:{dataConnectionId}", null, cancellationToken);
-
-        return Result<bool>.Success(true);
-    }
 }

tests/ScadaLink.ConfigurationDatabase.Tests/UnitTest1.cs

@@ -45,7 +45,6 @@ public class DbContextTests : IDisposable
         Assert.NotNull(_context.Areas);
         Assert.NotNull(_context.Sites);
         Assert.NotNull(_context.DataConnections);
-        Assert.NotNull(_context.SiteDataConnectionAssignments);
         Assert.NotNull(_context.DeploymentRecords);
         Assert.NotNull(_context.SystemArtifactDeploymentRecords);
         Assert.NotNull(_context.ExternalSystemDefinitions);
@@ -133,9 +132,11 @@ public class DbContextTests : IDisposable
     {
         var site = new Site("Site1", "SITE-001");
         var template = new Template("Template1");
-        var dataConn = new DataConnection("OpcConn", "OpcUa");
         _context.Sites.Add(site);
         _context.Templates.Add(template);
+        _context.SaveChanges();
+
+        var dataConn = new DataConnection("OpcConn", "OpcUa", site.Id);
         _context.DataConnections.Add(dataConn);
         _context.SaveChanges();
 
@@ -300,19 +301,18 @@ public class DbContextTests : IDisposable
     }
 
     [Fact]
-    public void SiteDataConnectionAssignment_CreatesBothForeignKeys()
+    public void DataConnection_BelongsToSite()
     {
         var site = new Site("Site1", "SITE-001");
-        var conn = new DataConnection("OpcConn", "OpcUa");
         _context.Sites.Add(site);
+        _context.SaveChanges();
+
+        var conn = new DataConnection("OpcConn", "OpcUa", site.Id);
         _context.DataConnections.Add(conn);
         _context.SaveChanges();
 
-        var assignment = new SiteDataConnectionAssignment { SiteId = site.Id, DataConnectionId = conn.Id };
+        var loaded = _context.DataConnections.Single(c => c.Name == "OpcConn");
-        _context.SiteDataConnectionAssignments.Add(assignment);
+        Assert.Equal(site.Id, loaded.SiteId);
-        _context.SaveChanges();
-
-        Assert.Single(_context.SiteDataConnectionAssignments.ToList());
     }
 
     [Fact]

tests/ScadaLink.DeploymentManager.Tests/ArtifactDeploymentServiceTests.cs

@@ -37,9 +37,8 @@ public class ArtifactDeploymentServiceTests
         _siteRepo.GetAllSitesAsync().Returns(new List<Site>());
 
         var service = CreateService();
-        var command = CreateCommand();
 
-        var result = await service.DeployToAllSitesAsync(command, "admin");
+        var result = await service.DeployToAllSitesAsync("admin");
 
         Assert.True(result.IsFailure);
         Assert.Contains("No sites", result.Error);

tests/ScadaLink.TemplateEngine.Tests/Flattening/FlatteningServiceTests.cs

@@ -200,7 +200,7 @@ public class FlatteningServiceTests
 
         var connections = new Dictionary<int, DataConnection>
         {
-            [100] = new("OPC-Server1", "OpcUa") { Id = 100, Configuration = "opc.tcp://localhost:4840" }
+            [100] = new("OPC-Server1", "OpcUa", 1) { Id = 100, Configuration = "opc.tcp://localhost:4840" }
         };
 
         var result = _sut.Flatten(

tests/ScadaLink.TemplateEngine.Tests/Services/SiteServiceTests.cs

@@ -93,44 +93,12 @@ public class SiteServiceTests
         _repoMock.Setup(r => r.SaveChangesAsync(It.IsAny<CancellationToken>()))
             .ReturnsAsync(1);
 
-        var result = await _sut.CreateDataConnectionAsync("OPC-Server1", "OpcUa", "{\"url\":\"opc.tcp://localhost\"}", "admin");
+        var result = await _sut.CreateDataConnectionAsync(1, "OPC-Server1", "OpcUa", "{\"url\":\"opc.tcp://localhost\"}", "admin");
 
         Assert.True(result.IsSuccess);
         Assert.Equal("OPC-Server1", result.Value.Name);
         Assert.Equal("OpcUa", result.Value.Protocol);
-    }
+        Assert.Equal(1, result.Value.SiteId);
-
-    [Fact]
-    public async Task AssignConnectionToSite_AlreadyAssigned_ReturnsFailure()
-    {
-        _repoMock.Setup(r => r.GetSiteByIdAsync(1, It.IsAny<CancellationToken>()))
-            .ReturnsAsync(new Site("S", "S1") { Id = 1 });
-        _repoMock.Setup(r => r.GetDataConnectionByIdAsync(100, It.IsAny<CancellationToken>()))
-            .ReturnsAsync(new DataConnection("Conn", "OpcUa") { Id = 100 });
-        _repoMock.Setup(r => r.GetSiteDataConnectionAssignmentAsync(1, 100, It.IsAny<CancellationToken>()))
-            .ReturnsAsync(new SiteDataConnectionAssignment { Id = 1, SiteId = 1, DataConnectionId = 100 });
-
-        var result = await _sut.AssignConnectionToSiteAsync(1, 100, "admin");
-
-        Assert.True(result.IsFailure);
-        Assert.Contains("already assigned", result.Error);
-    }
-
-    [Fact]
-    public async Task AssignConnectionToSite_Valid_Success()
-    {
-        _repoMock.Setup(r => r.GetSiteByIdAsync(1, It.IsAny<CancellationToken>()))
-            .ReturnsAsync(new Site("S", "S1") { Id = 1 });
-        _repoMock.Setup(r => r.GetDataConnectionByIdAsync(100, It.IsAny<CancellationToken>()))
-            .ReturnsAsync(new DataConnection("Conn", "OpcUa") { Id = 100 });
-        _repoMock.Setup(r => r.GetSiteDataConnectionAssignmentAsync(1, 100, It.IsAny<CancellationToken>()))
-            .ReturnsAsync((SiteDataConnectionAssignment?)null);
-        _repoMock.Setup(r => r.SaveChangesAsync(It.IsAny<CancellationToken>()))
-            .ReturnsAsync(1);
-
-        var result = await _sut.AssignConnectionToSiteAsync(1, 100, "admin");
-
-        Assert.True(result.IsSuccess);
     }
 
     [Fact]