19. Partial Objects

A partial object is an object whose state is not fully initialized after being reconstituted from the database and that is disconnected from the rest of its data. The following section will describe why partial objects are problematic and what the approach of Doctrine2 to this problem is.

Note

The partial object problem in general does not apply to methods or queries where you do not retrieve the query result as objects. Examples are: Query#getArrayResult(), Query#getScalarResult(), Query#getSingleScalarResult(), etc.

Warning

Use of partial objects is tricky. Fields that are not retrieved from the database will not be updated by the UnitOfWork even if they get changed in your objects. You can only promote a partial object to a fully-loaded object by calling EntityManager#refresh() or a DQL query with the refresh flag.

19.1. What is the problem?

In short, partial objects are problematic because they are usually objects with broken invariants. As such, code that uses these partial objects tends to be very fragile and either needs to “know” which fields or methods can be safely accessed or add checks around every field access or method invocation. The same holds true for the internals, i.e. the method implementations, of such objects. You usually simply assume the state you need in the method is available, after all you properly constructed this object before you pushed it into the database, right? These blind assumptions can quickly lead to null reference errors when working with such partial objects.

It gets worse with the scenario of an optional association (0..1 to 1). When the associated field is NULL, you don’t know whether this object does not have an associated object or whether it was simply not loaded when the owning object was loaded from the database.

These are reasons why many ORMs do not allow partial objects at all and instead you always have to load an object with all its fields (associations being proxied). One secure way to allow partial objects is if the programming language/platform allows the ORM tool to hook deeply into the object and instrument it in such a way that individual fields (not only associations) can be loaded lazily on first access. This is possible in Java, for example, through bytecode instrumentation. In PHP though this is not possible, so there is no way to have “secure” partial objects in an ORM with transparent persistence.

Doctrine, by default, does not allow partial objects. That means, any query that only selects partial object data and wants to retrieve the result as objects (i.e. Query#getResult()) will raise an exception telling you that partial objects are dangerous. If you want to force a query to return you partial objects, possibly as a performance tweak, you can use the partial keyword as follows:

<?php
$q = $em->createQuery("select partial u.{id,name} from MyApp\Domain\User u");

You can also get a partial reference instead of a proxy reference by calling:

<?php
$reference = $em->getPartialReference('MyApp\Domain\User', 1);

Partial references are objects with only the identifiers set as they are passed to the second argument of the getPartialReference() method. All other fields are null.

19.2. When should I force partial objects?

Mainly for optimization purposes, but be careful of premature optimization as partial objects lead to potentially more fragile code.

Fork me on GitHub