validation fix

Author: Faraz Ahmed

src/JsonApiDotNetCore/Services/QueryComposer.cs

@@ -31,7 +31,8 @@ public string Compose(IJsonApiContext jsonApiContext)
         private string ComposeSingleFilter(FilterQuery query)
         {
             var result = "&filter";
-            result += QueryConstants.OPEN_BRACKET + query.Attribute + QueryConstants.CLOSE_BRACKET + "=" + query.Operation + ":" + query.Value;
+            var operation = string.IsNullOrWhiteSpace(query.Operation) ? query.Operation : query.Operation + ":";
+            result += QueryConstants.OPEN_BRACKET + query.Attribute + QueryConstants.CLOSE_BRACKET + "=" + operation + query.Value;
             return result;
         }
     }

test/UnitTests/Services/QueryComposerTests.cs

@@ -42,9 +42,11 @@ public void Can_ComposeLessThan_FilterStringForUrl()
         {
             // arrange
             var filter = new FilterQuery("attribute", "value", "le");
+            var filter2 = new FilterQuery("attribute2", "value2", "");
             var querySet = new QuerySet();
             List<FilterQuery> filters = new List<FilterQuery>();
             filters.Add(filter);
+            filters.Add(filter2);
             querySet.Filters = filters;
 
             _jsonApiContext
@@ -55,7 +57,7 @@ public void Can_ComposeLessThan_FilterStringForUrl()
             // act
             var filterString = queryComposer.Compose(_jsonApiContext.Object);
             // assert
-            Assert.Equal("&filter[attribute]=le:value", filterString);
+            Assert.Equal("&filter[attribute]=le:value&filter[attribute2]=value2", filterString);
         }
 
         [Fact]