Merge pull request #1243 from st0012/fix-1242

Unscope includes when plucking linkage data

Author: lgebhardt

lib/jsonapi/resource_serializer.rb

@@ -418,6 +418,7 @@ def to_many_linkage(source, relationship)
             [obj.type.underscore.pluralize, obj.id]
           end
         else
+          assoc = assoc.unscope(:includes) if assoc.is_a?(ActiveRecord::Relation)
           assoc.pluck(:type, :id).map do |type, id|
             [type.underscore.pluralize, id]
           end