Fix for building LINQ Select clause on many-to-many includes (#869)

Author: Bart Koelman

src/Examples/JsonApiDotNetCoreExample/Models/Tag.cs

@@ -1,3 +1,5 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations.Schema;
 using JsonApiDotNetCore.Resources;
 using JsonApiDotNetCore.Resources.Annotations;
 
@@ -10,6 +12,11 @@ public class Tag : Identifiable
 
         [Attr]
         public TagColor Color { get; set; }
+
+        [NotMapped]
+        [HasManyThrough(nameof(ArticleTags))]
+        public ISet<Article> Articles { get; set; }
+        public ISet<ArticleTag> ArticleTags { get; set; }
     }
 
     public enum TagColor

src/JsonApiDotNetCore/Queries/Internal/QueryableBuilding/SelectClauseBuilder.cs

@@ -61,7 +61,7 @@ public Expression ApplySelect(IDictionary<ResourceFieldAttribute, QueryLayer> se
         private Expression CreateLambdaBodyInitializer(IDictionary<ResourceFieldAttribute, QueryLayer> selectors, ResourceContext resourceContext,
             LambdaScope lambdaScope, bool lambdaAccessorRequiresTestForNull)
         {
-            var propertySelectors = ToPropertySelectors(selectors, resourceContext, lambdaScope.Parameter.Type);
+            var propertySelectors = ToPropertySelectors(selectors, resourceContext, lambdaScope.Accessor.Type);
             MemberBinding[] propertyAssignments = propertySelectors.Select(selector => CreatePropertyAssignment(selector, lambdaScope)).Cast<MemberBinding>().ToArray();
 
             NewExpression newExpression = _resourceFactory.CreateNewExpression(lambdaScope.Accessor.Type);

test/JsonApiDotNetCoreExampleTests/IntegrationTests/Includes/IncludeTests.cs

@@ -329,6 +329,51 @@ await _testContext.RunOnDatabaseAsync(async dbContext =>
             responseDocument.Included[0].Attributes["name"].Should().Be(article.ArticleTags.Single().Tag.Name);
         }
 
+        [Fact]
+        public async Task Can_include_HasManyThrough_relationship_in_secondary_resource()
+        {
+            // Arrange
+            var article = new Article
+            {
+                Caption = "One",
+                ArticleTags = new HashSet<ArticleTag>
+                {
+                    new ArticleTag
+                    {
+                        Tag = new Tag
+                        {
+                            Name = "Hot"
+                        }
+                    }
+                }
+            };
+
+            await _testContext.RunOnDatabaseAsync(async dbContext =>
+            {
+                dbContext.Articles.Add(article);
+
+                await dbContext.SaveChangesAsync();
+            });
+
+            var route = $"/api/v1/articles/{article.StringId}/tags?include=articles";
+
+            // Act
+            var (httpResponse, responseDocument) = await _testContext.ExecuteGetAsync<Document>(route);
+
+            // Assert
+            httpResponse.Should().HaveStatusCode(HttpStatusCode.OK);
+
+            responseDocument.ManyData.Should().HaveCount(1);
+            responseDocument.ManyData[0].Type.Should().Be("tags");
+            responseDocument.ManyData[0].Id.Should().Be(article.ArticleTags.ElementAt(0).Tag.StringId);
+            responseDocument.ManyData[0].Attributes["name"].Should().Be(article.ArticleTags.Single().Tag.Name);
+
+            responseDocument.Included.Should().HaveCount(1);
+            responseDocument.Included[0].Type.Should().Be("articles");
+            responseDocument.Included[0].Id.Should().Be(article.StringId);
+            responseDocument.Included[0].Attributes["caption"].Should().Be(article.Caption);
+        }
+
         [Fact]
         public async Task Can_include_chain_of_HasOne_relationships()
         {