/**
* Relate a child object to this object.
* Uses the 'relationships' polymorphic table.
* Does not relate the objects if the relationship exists already.
* @param Model $child
* @return null|Relationship
*/
public function relate(Model $child)
{
if (Relationship::exists($this, $child)) {
return null;
}
return Relationship::relate($this, $child);
}