[Dbix-class] Avoiding foreign key constraint on a relationship

[W Snyder]

One of my DBIx::Class table classes has:

__PACKAGE__->table("adjacents");
__PACKAGE__->add_columns(
    "child_id"          => { ... },
    "parent_id"         => { ... },
);
__PACKAGE__->set_primary_key("child_id");
__PACKAGE__->base_has_one(parent_string => 'X::Schema::String',
	{'foreign.object_id' => 'self.parent_id'});

This results in the (MySQL) creation commands

CREATE TABLE `adjacents` (
  ...
  CONSTRAINT `fk_parent_id` FOREIGN KEY (`parent_id`)
       REFERENCES `strings` (`object_id`)

The problem is I want to eliminate this constraint from the
SQL creation but not from what DBIx accessors are made.
(Because the constraint is sometimes violated, and I need to
index it in a special way).

How do I create a relationship so I can call
$object->parent_string, but not have a CONSTRAINT added to
the database schema?

Thanks