DateTime doesn't have an IS method.... so I don't know how we'll use it....

Author: NullVoxPopuli

src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

@@ -113,24 +113,44 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
 
             var concreteType = typeof(TSource);
             var property = concreteType.GetProperty(filterQuery.FilteredAttribute.InternalAttributeName);
+            var op = filterQuery.FilterOperation;
 
             if (property == null)
                 throw new ArgumentException($"'{filterQuery.FilteredAttribute.InternalAttributeName}' is not a valid property of '{concreteType}'");
 
             try
             {
-                if (filterQuery.FilterOperation == FilterOperations.@in || filterQuery.FilterOperation == FilterOperations.nin)
+                if (op == FilterOperations.@in || op == FilterOperations.nin)
                 {
                     string[] propertyValues = filterQuery.PropertyValue.Split(',');
-                    var lambdaIn = ArrayContainsPredicate<TSource>(propertyValues, property.Name, filterQuery.FilterOperation);
+                    var lambdaIn = ArrayContainsPredicate<TSource>(propertyValues, property.Name, op);
 
                     return source.Where(lambdaIn);
                 }
+                else if (op == FilterOperations.@is || op == FilterOperations.isnot) {
+                    var parameter = Expression.Parameter(concreteType, "model");
+                    // {model.Id}
+                    var left = Expression.PropertyOrField(parameter, property.Name);
+                    var right = Expression.Constant(filterQuery.PropertyValue, typeof(string));
+
+                    var body = GetFilterExpressionLambda(left, right, op);
+                    var lambda = Expression.Lambda<Func<TSource, bool>>(body, parameter);
+
+                    return source.Where(lambda);
+                }
                 else
                 {   
                     var isNullabe = IsNullable(property.PropertyType);
                     var propertyValue = filterQuery.PropertyValue;
-                    var value = isNullabe && propertyValue == "" ? null : propertyValue;
+                    var value = propertyValue;
+
+                    if (op == FilterOperations.@isnot || op == FilterOperations.isnot)
+                    {
+                        if (isNullabe && propertyValue == "null") 
+                        {
+                            value = null;
+                        }
+                    }
 
                     // convert the incoming value to the target value type
                     // "1" -> 1
@@ -142,7 +162,7 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
                     // {1}
                     var right = Expression.Constant(convertedValue, property.PropertyType);
 
-                    var body = GetFilterExpressionLambda(left, right, filterQuery.FilterOperation);
+                    var body = GetFilterExpressionLambda(left, right, op);
 
                     var lambda = Expression.Lambda<Func<TSource, bool>>(body, parameter);
 
@@ -244,6 +264,14 @@ private static Expression GetFilterExpressionLambda(Expression left, Expression
                 case FilterOperations.ne:
                     body = Expression.NotEqual(left, right);
                     break;
+                case FilterOperations.isnot:
+                    // {model.Id != null}
+                    body = Expression.NotEqual(left, right);
+                    break;
+                case FilterOperations.@is:
+                    // {model.Id == null}
+                    body = Expression.Equal(left, right);
+                    break;
                 default:
                     throw new JsonApiException(500, $"Unknown filter operation {operation}");
             }

src/JsonApiDotNetCore/Internal/Query/FilterOperations.cs

@@ -11,6 +11,8 @@ public enum FilterOperations
         like = 5,
         ne = 6,
         @in = 7, // prefix with @ to use keyword
-        nin = 8
+        nin = 8,
+        @is = 9,
+        isnot = 10
     }
 }

test/JsonApiDotNetCoreExampleTests/Acceptance/TodoItemsControllerTests.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Net;
@@ -91,7 +92,30 @@ public async Task Can_Filter_TodoItems()
         }
 
         [Fact]
-        public async Task Can_Filter_TodoItems_Using_NotEqual_Operator()
+        public async Task Can_Filter_TodoItems_Using_IsNot_Operator()
+        {
+            // Arrange
+            var todoItem = _todoItemFaker.Generate();
+            todoItem.UpdatedDate = new DateTime();
+            _context.TodoItems.Add(todoItem);
+            _context.SaveChanges();
+
+            var httpMethod = new HttpMethod("GET");
+            var route = $"/api/v1/todo-items?filter[updated-date]=isnot:null";
+            var request = new HttpRequestMessage(httpMethod, route);
+
+            // Act
+            var response = await _fixture.Client.SendAsync(request);
+            var body = await response.Content.ReadAsStringAsync();
+            var deserializedBody = _fixture.GetService<IJsonApiDeSerializer>().DeserializeList<TodoItem>(body);
+
+            // Assert
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            Assert.NotEmpty(deserializedBody);
+        }
+
+        [Fact]
+        public async Task Can_Filter_TodoItems_Using_Is_Operator()
         {
             // Arrange
             var todoItem = _todoItemFaker.Generate();
@@ -100,7 +124,7 @@ public async Task Can_Filter_TodoItems_Using_NotEqual_Operator()
             _context.SaveChanges();
 
             var httpMethod = new HttpMethod("GET");
-            var route = $"/api/v1/todo-items?filter[updated-date]=ne:";
+            var route = $"/api/v1/todo-items?filter[updated-date]=is:null";
             var request = new HttpRequestMessage(httpMethod, route);
 
             // Act
@@ -110,7 +134,7 @@ public async Task Can_Filter_TodoItems_Using_NotEqual_Operator()
 
             // Assert
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
-            Assert.Empty(deserializedBody);
+            Assert.NotEmpty(deserializedBody);
         }
 
         [Fact]