Merge pull request #19680 from vngrs/make_join_model_name_const_private

Change join model name constant to private constant

activerecord/lib/active_record/associations.rb

@@ -1718,10 +1718,8 @@ def has_and_belongs_to_many(name, scope = nil, options = {}, &extension)
 
         join_model = builder.through_model
 
-        # FIXME: we should move this to the internal constants. Also people
-        # should never directly access this constant so I'm not happy about
-        # setting it.
         const_set join_model.name, join_model
+        private_constant join_model.name
 
         middle_reflection = builder.middle_reflection join_model