Remove the assignment for real this time.

cc:
https://github.com/rails/rails/commit/a11e2fcff8cdc9691b977d8330abe63024f8c732#commitcomment-6353614
and 0369dbfd

activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb

@@ -25,8 +25,8 @@ def self.build(lhs_class, name, options)
           class_name = options.fetch(:class_name) {
             model_name = name.to_s.camelize.singularize
 
-            if parent_name = lhs_class.parent_name
-              model_name.prepend("#{parent_name}::")
+            if lhs_class.parent_name
+              model_name.prepend("#{lhs_class.parent_name}::")
             end
 
             model_name