allow custom attribute filters

Author: jaredcnance

src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

@@ -90,7 +90,7 @@ public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> sourc
 
         public static IQueryable<TSource> Filter<TSource>(this IQueryable<TSource> source, AttrFilterQuery filterQuery)
         {
-            if (filterQuery == null)
+            if (filterQuery == null || filterQuery.IsAttribute == false)
                 return source;
 
             var concreteType = typeof(TSource);

src/JsonApiDotNetCore/Internal/Query/AttrFilterQuery.cs

@@ -18,7 +18,9 @@ public AttrFilterQuery(
             var attribute = GetAttribute(filterQuery.Attribute);
 
             if (attribute == null)
-                throw new JsonApiException(400, $"'{filterQuery.Attribute}' is not a valid attribute.");
+                return; // we don't want to throw...we should allow custom filter implementations
+
+            IsAttribute = true;
 
             if (attribute.IsFilterable == false)
                 throw new JsonApiException(400, $"Filter is not allowed for attribute '{attribute.PublicAttributeName}'.");
@@ -32,6 +34,12 @@ public AttrFilterQuery(
         public string PropertyValue { get; }
         public FilterOperations FilterOperation { get; }
 
+        /// <summary>
+        /// Whether or not the filter is an actual attribute on the model.
+        /// We use this to allow custom filters that have to be handled by the application.
+        /// </summary>
+        internal bool IsAttribute { get; set; }
+
         private AttrAttribute GetAttribute(string attribute) =>
             _jsonApiContext.RequestEntity.Attributes.FirstOrDefault(attr => attr.Is(attribute));
     }