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Collections Strategy

By default, every collection association has a strategy attached to it that is called during the hydrating and extracting phase. All those strategies extend from the class Doctrine\Laminas\Hydrator\Strategy\AbstractCollectionStrategy.

The library provides four strategies out of the box:

  1. Doctrine\Laminas\Hydrator\Strategy\AllowRemoveByValue: this is the default strategy; it removes old elements that are not in the new collection.
  2. Doctrine\Laminas\Hydrator\Strategy\AllowRemoveByReference: this is the default strategy if set to byReference; it removes old elements that are not in the new collection.
  3. Doctrine\Laminas\Hydrator\Strategy\DisallowRemoveByValue: this strategy does not remove old elements even if they are not in the new collection.
  4. Doctrine\Laminas\Hydrator\Strategy\DisallowRemoveByReference: this strategy does not remove old elements even if they are not in the new collection.

As a consequence, when using AllowRemove*, you need to define both adder (eg. addTags) and remover (eg. removeTags). On the other hand, when using the DisallowRemove* strategy, you must always define at least the adder, but the remover is optional (because elements are never removed).

The following table illustrates the difference between the two strategies

Strategy Initial collection Submitted collection Result
AllowRemove* A, B B, C B, C
DisallowRemove* A, B B, C A, B, C

The difference between ByValue and ByReference is that, when using strategies that end with ByReference, it won’t use the public API of your entity (adder and remover) - you don’t even need to define them - it will add and remove elements directly from the collection.

Changing the Strategy

Changing the strategy for collections is simple:

use Doctrine\Laminas\Hydrator\DoctrineObject as DoctrineHydrator;
use Doctrine\Laminas\Hydrator\Strategy;

$hydrator = new DoctrineHydrator($entityManager);
$hydrator->addStrategy('tags', new Strategy\DisallowRemoveByValue());

Note that you can also add strategies to non-collection fields.