This question got me today, my repositories should always return full objects? They can not return partial data (in an array for example)?
For example, I have the method getUserFriends(User $user) inside my repository Friends, in this method I execute the following DQL:
$dql = 'SELECT userFriend FROM Entities\User\Friend f JOIN f.friend userFriend WHERE f.user = ?0';
But this way I’m returning the users entities, containing all the properties, the generated SQL is a SELECT of all fields from the User table. But let’s say I just need the id and the name of the user friends, there would be more interesting (and quick) get just these values?
$dql = 'SELECT userFriend.id, userFriend.name FROM Entities\User\Friend f JOIN f.friend userFriend WHERE f.user = ?0';
These methods are executed in my service class.
From a database perspective, performance will not be that much affected by the number of fields, unless the number of rows to return is really huge (millions of rows, probably) : the hardest part for the db is to make the joints, and build the resultset from the tables.
From a php perspective, that depends on multiple factors, like the complexity and the number of objects created.
I would take the problem differently : I would profile and stress-test my code in order to see if performance is an issue or not, and decide to refactor only if needed (switching from doctrine to a hand-made model is time consuming, will the performance gain be worth it ?)
EDIT : and to answer your initial question : fetching complete objects will lead to easier caching if needed, and better data encapsulation. I would keep these until they represent a big performance issue.