public function get_primary_id() {
		if ( $this instanceof UsesWordPressPost ) {
			return $this->get_underlying_wp_object()->ID;
		}

		if ( $this instanceof UsesWordPressTerm ) {
			return $this->get_underlying_wp_object()->term_id;
		}

		if ( $this instanceof UsesCustomTable ) {
			return $this->get_attribute( $this->get_primary_key() );
		}

		// Model w/o wp_object not yet supported.
		throw new LogicException;
	}