Test for double linked middleware.

backend/extensions/Squidex.Extensions/Samples/Middleware/DoubleLinkedContentMiddleware.cs

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+// ==========================================================================
+//  Squidex Headless CMS
+// ==========================================================================
+//  Copyright (c) Squidex UG (haftungsbeschraenkt)
+//  All rights reserved. Licensed under the MIT license.
+// ==========================================================================
+
+using System.Linq;
+using System.Threading.Tasks;
+using Squidex.Domain.Apps.Core.Contents;
+using Squidex.Domain.Apps.Entities.Contents;
+using Squidex.Domain.Apps.Entities.Contents.Commands;
+using Squidex.Infrastructure;
+using Squidex.Infrastructure.Commands;
+using Squidex.Infrastructure.Json.Objects;
+
+namespace Squidex.Extensions.Samples.Middleware
+{
+    public sealed class DoubleLinkedContentMiddleware : ICustomCommandMiddleware
+    {
+        private readonly IContentLoader contentLoader;
+
+        public DoubleLinkedContentMiddleware(IContentLoader contentLoader)
+        {
+            this.contentLoader = contentLoader;
+        }
+
+        public async Task HandleAsync(CommandContext context, NextDelegate next)
+        {
+            await next(context);
+
+            if (context.Command is UpdateContent update && context.IsCompleted && update.SchemaId.Name == "source")
+            {
+                // After a change is made, the content is put to the command context.
+                var content = context.Result<IContentEntity>();
+
+                var contentPrevious =
+                    await contentLoader.GetAsync(
+                        content.AppId.Id,
+                        content.Id,
+                        content.Version - 1);
+
+                // The data might have been changed within the domain object. Therefore we do not use the data fro mthe command.
+                var oldReferenceId = GetReference(contentPrevious?.Data);
+                var newReferenceId = GetReference(content.Data);
+
+                // If nothing has been changed we can just stop here.
+                if (newReferenceId == oldReferenceId)
+                {
+                    return;
+                }
+
+                if (oldReferenceId != null)
+                {
+                    var oldReferenced = await contentLoader.GetAsync(content.AppId.Id, DomainId.Create(oldReferenceId));
+
+                    if (oldReferenced != null)
+                    {
+                        var data = oldReferenced.Data.Clone();
+
+                        // Remove the reference from the old referenced content.
+                        data.Remove("referencing");
+
+                        await UpdateReferencing(context, oldReferenced, data);
+                    }
+                }
+
+                if (newReferenceId != null)
+                {
+                    var newReferenced = await contentLoader.GetAsync(content.AppId.Id, DomainId.Create(newReferenceId));
+
+                    if (newReferenced != null)
+                    {
+                        var data = newReferenced.Data.Clone();
+
+                        // Add the reference to the new referenced content.
+                        data["referencing"] = new ContentFieldData
+                        {
+                            ["iv"] = JsonValue.Array(content.Id)
+                        };
+
+                        await UpdateReferencing(context, newReferenced, data);
+                    }
+                }
+            }
+        }
+
+        private static async Task UpdateReferencing(CommandContext context, IContentEntity reference, ContentData data)
+        {
+            await context.CommandBus.PublishAsync(new UpdateContent
+            {
+                AppId = reference.AppId,
+                SchemaId = reference.SchemaId,
+                ContentId = reference.Id,
+                DoNotScript = true,
+                DoNotValidate = true,
+                Data = data,
+                // Also set the expected version, otherwise it will be overriden with the version from the request.
+                ExpectedVersion = reference.Version
+            });
+        }
+
+        private static string GetReference(ContentData data)
+        {
+            if (data != null && data.TryGetValue("reference", out ContentFieldData fieldData))
+            {
+                return fieldData.Values.OfType<JsonArray>().SelectMany(x => x).SingleOrDefault()?.ToString();
+            }
+
+            return null;
+        }
+    }
+}