Included Data Should Only Contain Unique Entries

src/JsonApiDotNetCore/Builders/DocumentBuilder.cs

@@ -188,7 +188,8 @@ private List<DocumentData> GetIncludedEntities(ContextEntity contextEntity, IIde
         private void AddIncludedEntity(List<DocumentData> entities, IIdentifiable entity)
         {
             var includedEntity = GetIncludedEntity(entity);
-            if(includedEntity != null)
+
+            if(includedEntity != null && !entities.Any(doc => doc.Id == includedEntity.Id && doc.Type == includedEntity.Type))
                 entities.Add(includedEntity);
         }
 

src/JsonApiDotNetCoreExample/Data/AppDbContext.cs

@@ -9,6 +9,19 @@ public AppDbContext(DbContextOptions<AppDbContext> options)
         : base(options)
         { }
 
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<TodoItem>()
+                .HasOne(t => t.Assignee)
+                .WithMany(p => p.AssignedTodoItems)
+                .HasForeignKey(t => t.AssigneeId);
+
+            modelBuilder.Entity<TodoItem>()
+                .HasOne(t => t.Owner)
+                .WithMany(p => p.TodoItems)
+                .HasForeignKey(t => t.OwnerId);
+        }
+
         public DbSet<TodoItem> TodoItems { get; set; }
         public DbSet<Person> People { get; set; }
         public DbSet<TodoItemCollection> TodoItemCollections { get; set; }

src/JsonApiDotNetCoreExample/JsonApiDotNetCoreExample.csproj

@@ -28,6 +28,7 @@
     <PackageReference Include="Microsoft.Extensions.Logging.Debug" Version="1.1.1" />
     <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="1.1.1" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="1.0.0" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="1.1.0" />
     <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="1.1.0" />
     <PackageReference Include="DotNetCoreDocs" Version="0.4.0" />
   </ItemGroup>

src/JsonApiDotNetCoreExample/Migrations/20170330020650_AddAssignedTodoItems.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+namespace JsonApiDotNetCoreExample.Migrations
+{
+    public partial class AddAssignedTodoItems : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "AssigneeId",
+                table: "TodoItems",
+                nullable: true);
+
+            migrationBuilder.CreateIndex(
+                name: "IX_TodoItems_AssigneeId",
+                table: "TodoItems",
+                column: "AssigneeId");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_TodoItems_People_AssigneeId",
+                table: "TodoItems",
+                column: "AssigneeId",
+                principalTable: "People",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Restrict);
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_TodoItems_People_AssigneeId",
+                table: "TodoItems");
+
+            migrationBuilder.DropIndex(
+                name: "IX_TodoItems_AssigneeId",
+                table: "TodoItems");
+
+            migrationBuilder.DropColumn(
+                name: "AssigneeId",
+                table: "TodoItems");
+        }
+    }
+}

src/JsonApiDotNetCoreExample/Migrations/AppDbContextModelSnapshot.cs

@@ -35,6 +35,8 @@ protected override void BuildModel(ModelBuilder modelBuilder)
                     b.Property<int>("Id")
                         .ValueGeneratedOnAdd();
 
+                    b.Property<int?>("AssigneeId");
+
                     b.Property<Guid?>("CollectionId");
 
                     b.Property<string>("Description");
@@ -45,6 +47,8 @@ protected override void BuildModel(ModelBuilder modelBuilder)
 
                     b.HasKey("Id");
 
+                    b.HasIndex("AssigneeId");
+
                     b.HasIndex("CollectionId");
 
                     b.HasIndex("OwnerId");
@@ -70,6 +74,10 @@ protected override void BuildModel(ModelBuilder modelBuilder)
 
             modelBuilder.Entity("JsonApiDotNetCoreExample.Models.TodoItem", b =>
                 {
+                    b.HasOne("JsonApiDotNetCoreExample.Models.Person", "Assignee")
+                        .WithMany("AssignedTodoItems")
+                        .HasForeignKey("AssigneeId");
+
                     b.HasOne("JsonApiDotNetCoreExample.Models.TodoItemCollection", "Collection")
                         .WithMany("TodoItems")
                         .HasForeignKey("CollectionId");

src/JsonApiDotNetCoreExample/Models/Person.cs

@@ -15,6 +15,9 @@ public class Person : Identifiable, IHasMeta
 
         [HasMany("todo-items")]
         public virtual List<TodoItem> TodoItems { get; set; }
+
+        [HasMany("assigned-todo-items")]
+        public virtual List<TodoItem> AssignedTodoItems { get; set; }
 
         [HasMany("todo-item-collections")]
         public virtual List<TodoItemCollection> TodoItemCollections { get; set; }

src/JsonApiDotNetCoreExample/Models/TodoItem.cs

@@ -12,11 +12,15 @@ public class TodoItem : Identifiable
         public long Ordinal { get; set; }
 
         public int? OwnerId { get; set; }
+        public int? AssigneeId { get; set; }
         public Guid? CollectionId { get; set; }
 
         [HasOne("owner")]
         public virtual Person Owner { get; set; }
 
+        [HasOne("assignee")]
+        public virtual Person Assignee { get; set; }
+
         [HasOne("collection")]
         public virtual TodoItemCollection Collection { get; set; }
     }

test/JsonApiDotNetCoreExampleTests/Acceptance/Spec/DocumentTests/Included.cs

@@ -134,6 +134,40 @@ public async Task GET_Included_Contains_SideloadedData_OneToMany()
             Assert.Equal(documents.Data.Count, documents.Included.Count);
         }
 
+        [Fact]
+        public async Task GET_Included_DoesNot_Duplicate_Records_ForMultipleRelationshipsOfSameType()
+        {
+            // arrange
+            _context.People.RemoveRange(_context.People); // ensure all people have todo-items
+            _context.TodoItems.RemoveRange(_context.TodoItems);
+            var person = _personFaker.Generate();
+            var todoItem = _todoItemFaker.Generate();
+            todoItem.Owner = person;
+            todoItem.Assignee = person;
+            _context.TodoItems.Add(todoItem);
+            _context.SaveChanges();
+
+            var builder = new WebHostBuilder()
+                .UseStartup<Startup>();
+
+            var httpMethod = new HttpMethod("GET");
+            var route = $"/api/v1/todo-items/{todoItem.Id}?include=owner&include=assignee";
+
+            var server = new TestServer(builder);
+            var client = server.CreateClient();
+            var request = new HttpRequestMessage(httpMethod, route);
+
+            // act
+            var response = await client.SendAsync(request);
+            var documents = JsonConvert.DeserializeObject<Document>(await response.Content.ReadAsStringAsync());
+            var data = documents.Data;
+
+            // assert
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            Assert.NotEmpty(documents.Included);
+            Assert.Equal(1, documents.Included.Count);
+        }
+
         [Fact]
         public async Task GET_ById_Included_Contains_SideloadedData_ForOneToMany()
         {