fix(GetOpProcessor): #340.1 not properly handling HasMany relationships

Author: jaredcnance

Diff for: src/JsonApiDotNetCore/Builders/DocumentBuilder.cs

@@ -138,7 +138,6 @@ public DocumentData GetData(ContextEntity contextEntity, IIdentifiable entity, I
 
             return data;
         }
-
         private bool ShouldIncludeAttribute(AttrAttribute attr, object attributeValue)
         {
             return OmitNullValuedAttribute(attr, attributeValue) == false

Diff for: src/JsonApiDotNetCore/Services/Operations/Processors/GetOpProcessor.cs

@@ -1,3 +1,4 @@
+using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
@@ -83,10 +84,10 @@ public async Task<Operation> ProcessAsync(Operation operation)
             };
 
             operationResult.Data = string.IsNullOrWhiteSpace(operation.Ref.Id)
-            ? await GetAllAsync(operation)
-            : string.IsNullOrWhiteSpace(operation.Ref.Relationship)
-                ? await GetByIdAsync(operation)
-                : await GetRelationshipAsync(operation);
+                ? await GetAllAsync(operation)
+                : string.IsNullOrWhiteSpace(operation.Ref.Relationship)
+                    ? await GetByIdAsync(operation)
+                    : await GetRelationshipAsync(operation);
 
             return operationResult;
         }
@@ -135,11 +136,38 @@ private async Task<object> GetRelationshipAsync(Operation operation)
             // when no generic parameter is available
             var relationshipType = _contextGraph.GetContextEntity(operation.GetResourceTypeName())
                 .Relationships.Single(r => r.Is(operation.Ref.Relationship)).Type;
+
             var relatedContextEntity = _jsonApiContext.ContextGraph.GetContextEntity(relationshipType);
 
-            var doc = _documentBuilder.GetData(relatedContextEntity, result as IIdentifiable); // TODO: if this is safe, then it should be cast in the GetRelationshipAsync call
+            if (result == null)
+                return null;
+
+            if (result is IIdentifiable singleResource)
+                return GetData(relatedContextEntity, singleResource);
 
-            return doc;
+            if (result is IEnumerable multipleResults)
+                return GetData(relatedContextEntity, multipleResults);
+
+            throw new JsonApiException(500,
+                $"An unexpected type was returned from '{_getRelationship.GetType()}.{nameof(IGetRelationshipService<T, TId>.GetRelationshipAsync)}'.",
+                detail: $"Type '{result.GetType()} does not implement {nameof(IIdentifiable)} nor {nameof(IEnumerable<IIdentifiable>)}'");
+        }
+
+        private DocumentData GetData(ContextEntity contextEntity, IIdentifiable singleResource)
+        {
+            return _documentBuilder.GetData(contextEntity, singleResource);
+        }
+
+        private List<DocumentData> GetData(ContextEntity contextEntity, IEnumerable multipleResults)
+        {
+            var resources = new List<DocumentData>();
+            foreach (var singleResult in multipleResults)
+            {
+                if (singleResult is IIdentifiable resource)
+                    resources.Add(_documentBuilder.GetData(contextEntity, resource));
+            }
+
+            return resources;
         }
 
         private TId GetReferenceId(Operation operation) => TypeHelper.ConvertType<TId>(operation.Ref.Id);

Diff for: test/OperationsExampleTests/Get/GetRelationshipTests.cs

@@ -16,7 +16,7 @@ public class GetRelationshipTests : Fixture, IDisposable
         private readonly Faker _faker = new Faker();
 
         [Fact]
-        public async Task Can_Get_Article_Author()
+        public async Task Can_Get_HasOne_Relationship()
         {
             // arrange
             var context = GetService<AppDbContext>();
@@ -48,5 +48,40 @@ public async Task Can_Get_Article_Author()
             Assert.Equal(author.Id.ToString(), resourceObject.Id);
             Assert.Equal("authors", resourceObject.Type);
         }
+
+        [Fact]
+        public async Task Can_Get_HasMany_Relationship()
+        {
+            // arrange
+            var context = GetService<AppDbContext>();
+            var author = AuthorFactory.Get();
+            var article = ArticleFactory.Get();
+            article.Author = author;
+            context.Articles.Add(article);
+            context.SaveChanges();
+
+            var content = new
+            {
+                operations = new[] {
+                    new Dictionary<string, object> {
+                        { "op", "get"},
+                        { "ref",  new { type = "authors", id = author.StringId, relationship = "articles" } }
+                    }
+                }
+            };
+
+            // act
+            var (response, data) = await PatchAsync<OperationsDocument>("api/bulk", content);
+
+            // assert
+            Assert.NotNull(response);
+            Assert.NotNull(data);
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            Assert.Single(data.Operations);
+
+            var resourceObject = data.Operations.Single().DataList.Single();
+            Assert.Equal(article.Id.ToString(), resourceObject.Id);
+            Assert.Equal("articles", resourceObject.Type);
+        }
     }
 }