Like filter

README.md

@@ -285,6 +285,7 @@ identifier):
 ?filter[attribute]=gt:value
 ?filter[attribute]=le:value
 ?filter[attribute]=ge:value
+?filter[attribute]=like:value
 ```
 
 ### Sorting

src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

@@ -86,7 +86,7 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
                 // {1}
                 var right = Expression.Constant(convertedValue, property.PropertyType);
 
-                var body = Expression.Equal(left, right);
+                Expression body;
                 switch (filterQuery.FilterOperation)
                 {
                     case FilterOperations.eq:
@@ -109,6 +109,12 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
                         // {model.Id <= 1}
                         body = Expression.GreaterThanOrEqual(left, right);
                         break;
+                    case FilterOperations.like:
+                        // {model.Id <= 1}
+                        body = Expression.Call(left, "Contains", null, right);
+                        break;
+                    default:
+                        throw new JsonApiException("500", $"Unknown filter operation {filterQuery.FilterOperation}");
                 }
 
                 var lambda = Expression.Lambda<Func<TSource, bool>>(body, parameter);

src/JsonApiDotNetCore/Internal/Query/FilterOperations.cs

@@ -6,6 +6,7 @@ public enum FilterOperations
         lt = 1,
         gt = 2,
         le = 3,
-        ge = 4
+        ge = 4,
+        like = 5
     }
 }

src/JsonApiDotNetCore/Internal/Query/QuerySet.cs

@@ -83,26 +83,29 @@ private FilterQuery ParseFilterOperation(AttrAttribute attribute, string value)
             if(value.Length < 3)
                 return new FilterQuery(attribute, value, FilterOperations.eq);
 
-            var prefix = value.Substring(0, 3);
+            var operation = value.Split(':');
 
-            if(prefix[2] != ':')
+            if(operation.Length == 1)
                 return new FilterQuery(attribute, value, FilterOperations.eq);
 
             // remove prefix from value
-            value = value.Substring(3, value.Length - 3);
+            var prefix = operation[0];
+            value = operation[1];
 
             switch(prefix)
             {
-                case "eq:":
+                case "eq":
                     return new FilterQuery(attribute, value, FilterOperations.eq);
-                case "lt:":
+                case "lt":
                     return new FilterQuery(attribute, value, FilterOperations.lt);
-                case "gt:":
+                case "gt":
                     return new FilterQuery(attribute, value, FilterOperations.gt);
-                case "le:":
+                case "le":
                     return new FilterQuery(attribute, value, FilterOperations.le);
-                case "ge:":
+                case "ge":
                     return new FilterQuery(attribute, value, FilterOperations.ge);
+                case "like":
+                    return new FilterQuery(attribute, value, FilterOperations.like);
             }
 
             throw new JsonApiException("400", $"Invalid filter prefix '{prefix}'");

test/JsonApiDotNetCoreExampleTests/Acceptance/TodoItemsControllerTests.cs

@@ -124,6 +124,36 @@ public async Task Can_Filter_TodoItems()
                 Assert.Equal(todoItem.Ordinal, todoItemResult.Ordinal);
         }
 
+        [Fact]
+        public async Task Can_Filter_TodoItems_Using_Like_Operator()
+        {
+            // Arrange
+            var todoItem = _todoItemFaker.Generate();
+            todoItem.Ordinal = 999999;
+            _context.TodoItems.Add(todoItem);
+            _context.SaveChanges();
+            var substring = todoItem.Description.Substring(1, todoItem.Description.Length - 2);
+
+            var httpMethod = new HttpMethod("GET");
+            var route = $"/api/v1/todo-items?filter[description]=like:{substring}";
+
+            var description = new RequestProperties("Filter TodoItems Where Attribute Like", new Dictionary<string, string> {
+                { "?filter[...]=", "Filter on attribute" }
+            });
+
+            // Act
+            var response = await _fixture.MakeRequest<TodoItem>(description, httpMethod, route);
+            var body = await response.Content.ReadAsStringAsync();
+            var deserializedBody = JsonApiDeSerializer.DeserializeList<TodoItem>(body, _jsonApiContext);
+
+            // Assert
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            Assert.NotEmpty(deserializedBody);
+
+            foreach (var todoItemResult in deserializedBody)
+                Assert.Contains(substring, todoItem.Description);
+        }
+
         [Fact]
         public async Task Can_Sort_TodoItems_By_Ordinal_Ascending()
         {