public function foreignKey($primaryKey) {
		$result = [];
		$entity = $this->_classes['entity'];
		$keys = ($this->type() === 'belongsTo') ? array_flip($this->key()) : $this->key();
		$primaryKey = ($primaryKey instanceof $entity) ? $primaryKey->to('array') : $primaryKey;

		foreach ($keys as $key => $foreignKey) {
			$result[$foreignKey] = $primaryKey[$key];
		}
		return $result;
	}