protected _relation ( string $classType, string $key, array $data, array $options = [] ) : object | ||
$classType | string | The type of class to create, either `'entity'` or `'set'`. |
$key | string | The key name to which the related object is assigned. |
$data | array | The internal data of the related object. |
$options | array | Any other options to pass when instantiating the related object. |
return | object | Returns a new `Document` object instance. |