Added value types support to the filters isnull & isnotnull. (#549)

Author: Sergey Koshel

src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

@@ -170,11 +170,29 @@ private static Expression GetFilterExpressionLambda(Expression left, Expression
                     break;
                 case FilterOperations.isnotnull:
                     // {model.Id != null}
-                    body = Expression.NotEqual(left, right);
+                    if (left.Type.IsValueType &&
+                        !(left.Type.IsGenericType && left.Type.GetGenericTypeDefinition() == typeof(Nullable<>)))
+                    {
+                        var nullableType = typeof(Nullable<>).MakeGenericType(left.Type);
+                        body = Expression.NotEqual(Expression.Convert(left, nullableType), right);
+                    }
+                    else
+                    {
+                        body = Expression.NotEqual(left, right);
+                    }
                     break;
                 case FilterOperations.isnull:
                     // {model.Id == null}
-                    body = Expression.Equal(left, right);
+                    if (left.Type.IsValueType &&
+                        !(left.Type.IsGenericType && left.Type.GetGenericTypeDefinition() == typeof(Nullable<>)))
+                    {
+                        var nullableType = typeof(Nullable<>).MakeGenericType(left.Type);
+                        body = Expression.Equal(Expression.Convert(left, nullableType), right);
+                    }
+                    else
+                    {
+                        body = Expression.Equal(left, right);
+                    }
                     break;
                 default:
                     throw new JsonApiException(500, $"Unknown filter operation {operation}");

test/JsonApiDotNetCoreExampleTests/Acceptance/TodoItemsControllerTests.cs

@@ -175,6 +175,36 @@ public async Task Can_Filter_TodoItems_Using_IsNotNull_Operator()
             Assert.All(todoItems, t => Assert.NotNull(t.UpdatedDate));
         }
 
+        [Fact]
+        public async Task Can_Filter_TodoItems_ByParent_Using_IsNotNull_Operator()
+        {
+            // Arrange
+            var todoItem = _todoItemFaker.Generate();
+            todoItem.Assignee = new Person();
+
+            var otherTodoItem = _todoItemFaker.Generate();
+            otherTodoItem.Assignee = null;
+
+            _context.TodoItems.AddRange(new[] { todoItem, otherTodoItem });
+            _context.SaveChanges();
+
+            var httpMethod = new HttpMethod("GET");
+            var route = $"/api/v1/todo-items?filter[assignee.id]=isnotnull:";
+            var request = new HttpRequestMessage(httpMethod, route);
+
+            // Act
+            var response = await _fixture.Client.SendAsync(request);
+
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+
+            var body = await response.Content.ReadAsStringAsync();
+            var todoItems = _fixture.GetService<IJsonApiDeSerializer>().DeserializeList<TodoItem>(body);
+
+            // Assert
+            Assert.NotEmpty(todoItems);
+            Assert.All(todoItems, t => Assert.NotNull(t.Assignee));
+        }
+
         [Fact]
         public async Task Can_Filter_TodoItems_Using_IsNull_Operator()
         {
@@ -205,6 +235,36 @@ public async Task Can_Filter_TodoItems_Using_IsNull_Operator()
             Assert.All(todoItems, t => Assert.Null(t.UpdatedDate));
         }
 
+        [Fact]
+        public async Task Can_Filter_TodoItems_ByParent_Using_IsNull_Operator()
+        {
+            // Arrange
+            var todoItem = _todoItemFaker.Generate();
+            todoItem.Assignee = null;
+
+            var otherTodoItem = _todoItemFaker.Generate();
+            otherTodoItem.Assignee = new Person();
+
+            _context.TodoItems.AddRange(new[] { todoItem, otherTodoItem });
+            _context.SaveChanges();
+
+            var httpMethod = new HttpMethod("GET");
+            var route = $"/api/v1/todo-items?filter[assignee.id]=isnull:";
+            var request = new HttpRequestMessage(httpMethod, route);
+
+            // Act
+            var response = await _fixture.Client.SendAsync(request);
+
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+
+            var body = await response.Content.ReadAsStringAsync();
+            var todoItems = _fixture.GetService<IJsonApiDeSerializer>().DeserializeList<TodoItem>(body);
+
+            // Assert
+            Assert.NotEmpty(todoItems);
+            Assert.All(todoItems, t => Assert.Null(t.Assignee));
+        }
+
         [Fact]
         public async Task Can_Filter_TodoItems_Using_Like_Operator()
         {