Only initialize from ActiveRecord model properties

Author: lgebhardt

lib/jsonapi/resource.rb

@@ -674,10 +674,12 @@ def _add_relationship(klass, *attrs)
         attrs.each do |attr|
           check_reserved_relationship_name(attr)
 
-          # Initialize from the model's properties.
-          model_association = _model_class.reflect_on_association(attr)
-          if model_association
-            options[:class_name] ||= model_association.class_name
+          # Initialize from an ActiveRecord model's properties
+          if _model_class < ActiveRecord::Base
+            model_association = _model_class.reflect_on_association(attr)
+            if model_association
+              options[:class_name] ||= model_association.class_name
+            end
           end
 
           @_relationships[attr] = relationship = klass.new(attr, options)