Partial Objects

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 this became possible in PHP 8.4 through native lazy ghost objects. See Transparent lazy loading on PHP 8.4 for details.

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.

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.

Transparent lazy loading on PHP 8.4

When native lazy objects are enabled (PHP 8.4+), partial objects returned by DQL queries behave as lazy ghosts: only the fields listed in the PARTIAL clause are loaded eagerly from the database. All other scalar fields remain uninitialized in the ghost object.

On first access to any uninitialized field, Doctrine automatically issues a SELECT to load the full entity from the database, transparently initializing the ghost. From that point the object is indistinguishable from one loaded by a normal (non-partial) query.

<?php// Only id and name are fetched initially.$q = $em->createQuery("SELECT PARTIAL u.{id, name} FROM MyApp\Domain\User u");$user = $q->getSingleResult();// Accessing a loaded field — no additional query fired.echo $user->name;// Accessing a non-loaded field — Doctrine fetches the full entity now.echo $user->email; // triggers SELECT, then returns the value

This means the usual caveats about partial objects (broken invariants, null-vs-not-loaded ambiguity) no longer apply when native lazy objects are enabled: the object always presents complete, correct state to the caller, and the initial partial load serves purely as a performance optimisation that defers the cost of fetching remaining columns until (and unless) they are actually needed.

Mutating a loaded field before the lazy initializer fires is safe. The in-memory value is preserved and not overwritten when the ghost is eventually initialized:

<?php$user = $em->createQuery("SELECT PARTIAL u.{id, name} FROM MyApp\Domain\User u")           ->getSingleResult();// 'name' was loaded; change it before accessing any unloaded field.$user->name = 'Bob';// Accessing 'email' triggers the lazy load, but 'name' is NOT reset// to the database value — the in-memory change is preserved.echo $user->email;echo $user->name; // 'Bob'