Use available method rather than macro name checking

activerecord/lib/active_record/associations/through_association.rb

@@ -77,7 +77,7 @@ def foreign_key_present?
         end
 
         def ensure_mutable
-          if source_reflection.macro != :belongs_to
+          unless source_reflection.belongs_to?
             raise HasManyThroughCantAssociateThroughHasOneOrManyReflection.new(owner, reflection)
           end
         end