Working with Large Collections in Doctrine2

If you access a collection of Entity A pointing to Entity B, Doctrine2 always initializes the complete collection for you. For small collections up to around 100 entities this won’t be a problem, however as soon as collections get (much) bigger than this you can get into serious trouble.

By default Doctrine2 can only optimize adding new entities to a collection for you. This operation does not initialize the collection. This will only get you bigger collections though, reading them is still a pain.

We already got requests from several development teams for better functionality in this regard and we are planning to add a solution to this problem that is not constraining your domain model with technical blurp. However this solution is currently on our schedule for the 2.1 release of Doctrine only.

Until then I wrote a very little extension for Doctrine2 that allows you to work with large collections. It has two methods that compute the following data for any given PersistentCollection:

  • Total Number of Elements in the Collection
  • A slice of entities from the collection using a sql limit (or alternative)

You can get this extension from the DoctrineExtensions Github repository.

Working with a LargeCollection

The LargeCollection class is a handler to work with large PersistentCollections. You can instantiate it by passing an EntityManager instance:

<?php
use DoctrineExtensions\LargeCollections\LargeCollection;

$lc = new LargeCollection($em);

**NOTE**

LargeCollection only works with instances of
``PersistentCollection``, not with other implementations of the
``Doctrine\Common\Collections\Collection`` interface. That means
that you can only pass collections to it, whose owning entities
have been persisted before or are retrieved from the
EntityManager.

You can compute the total number of elements in a given collection by passing it to the count method:

<?php
$size = $lc->count($article->getComments());

You can retrieve a slice of entities from the collection by calling:

<?php
$slice = $lc->getSliceQuery($article->getComments(), $limit = 30);

As you can see this is very simple to use, but also missing some bits:

  • In your domain models you sometimes don’t want to return the Collections instance but call toArray() to encapsulate the Collections API inside the Entity. For this two new methods are required to access to the persistent collections from the inside of an entity.
  • The remove, removeElement, contains and containsKey methods could also be added to the large collection handler, making direct calls to the underlying UnitOfWork API.
  • A method that returns an IterableResult for any given collection. This would allow to iterate the complete collection on a row-by-row basis, which would eliminate possible max memory problems compared to the complete hydration of a collection.
  • Methods link()/unlink() like described in DDC-128

I hope I got your attention and maybe someone has an interest in extending the LargeCollection a little bit more.

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