diff --git a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb b/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
index afbf6dfb944fb342192fc6d0c4011df18967de72..44486ad75830e2390b739b9a59a3e24f4b108ed0 100644
--- a/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb
+++ b/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