def _register_existing_model(
        self, value: "Model", child: "Model", to_register: bool
    ) -> "Model":
        """
        Takes already created instance and registers it for parent.
        Registration is mutual, so children have also reference to parent.

        Used in reverse FK relations and normal FK for single models.

        :param value: already instantiated Model
        :type value: Model
        :param child: child/ related Model
        :type child: Model
        :param to_register: flag if the relation should be set in RelationshipManager
        :type to_register: bool
        :return: (if needed) registered Model
        :rtype: Model
        """
        if to_register:
            self.register_relation(model=value, child=child)
        return value

        Takes already created instance and registers it for parent.
        Registration is mutual, so children have also reference to parent.

        Used in reverse FK relations and normal FK for single models.

        :param value: already instantiated Model
        :type value: Model
        :param child: child/ related Model
        :type child: Model
        :param to_register: flag if the relation should be set in RelationshipManager
        :type to_register: bool
        :return: (if needed) registered Model
        :rtype: Model
        
ormar/fields/foreign_key.py