An entity can be made persistent by passing it to the EntityManager#persist($entity) method. By applying the persist operation on some entity, that entity becomes MANAGED, which means that its persistence is from now on managed by an EntityManager. As a result the persistent state of such an entity will subsequently be properly synchronized with the database when EntityManager#flush() is invoked.

Invoking the persist method on an entity does NOT cause an immediate SQL INSERT to be issued on the database. Doctrine applies a strategy called "transactional write-behind", which means that it will delay most SQL commands until EntityManager#flush() is invoked which will then issue all necessary SQL statements to synchronize your objects with the database in the most efficient way and a single, short transaction, taking care of maintaining referential integrity.

Example:

<?php

$user = new User;
$user->setName('Mr.Right');
$em->persist($user);
$em->flush();

Generated entity identifiers / primary keys are guaranteed to be available after the next successful flush operation that involves the entity in question. You can not rely on a generated identifier to be available directly after invoking persist. The inverse is also true. You can not rely on a generated identifier being not available after a failed flush operation.

The semantics of the persist operation, applied on an entity X, are as follows:

• If X is a new entity, it becomes managed. The entity X will be entered into the database as a result of the flush operation.
• If X is a preexisting managed entity, it is ignored by the persist operation. However, the persist operation is cascaded to entities referenced by X, if the relationships from X to these other entities are mapped with cascade=PERSIST or cascade=ALL (see "Transitive Persistence").
• If X is a removed entity, it becomes managed.
• If X is a detached entity, an exception will be thrown on flush.

An entity can be removed from persistent storage by passing it to the EntityManager#remove($entity) method. By applying the remove operation on some entity, that entity becomes REMOVED, which means that its persistent state will be deleted once EntityManager#flush() is invoked.

Just like persist, invoking remove on an entity does NOT cause an immediate SQL DELETE to be issued on the database. The entity will be deleted on the next invocation of EntityManager#flush() that involves that entity.

Example:

<?php

$em->remove($user);
$em->flush();

The semantics of the remove operation, applied to an entity X are as follows:

• If X is a new entity, it is ignored by the remove operation. However, the remove operation is cascaded to entities referenced by X, if the relationship from X to these other entities is mapped with cascade=REMOVE or cascade=ALL (see "Transitive Persistence").
• If X is a managed entity, the remove operation causes it to become removed. The remove operation is cascaded to entities referenced by X, if the relationships from X to these other entities is mapped with cascade=REMOVE or cascade=ALL (see "Transitive Persistence").
• If X is a detached entity, an InvalidArgumentException will be thrown.
• If X is a removed entity, it is ignored by the remove operation.
• A removed entity X will be removed from the database as a result of the flush operation.

After an entity has been removed its in-memory state is the same as before the removal, except for generated identifiers.

Removing an entity will also automatically delete any exisiting records in many-to-many join tables that link this entity. The action taken depends on the value of the @joinColumn mapping attribute "onDelete". Either Doctrine issues a dedicated DELETE statement for records of each join table or it depends on the foreign key semantics of onDelete="CASCADE".

Deleting an object with all its associated objects can be achieved in multiple ways with very different performance impacts.

1. If an association is marked as CASCADE=REMOVE Doctrine 2 will fetch this association. If its a Single association it will pass this entity to ´EntityManager#remove(). If the association is a collection, Doctrine will loop over all its elements and pass them toEntityManager#remove()`. In both cases the cascade remove semantics are applied recursively. For large object graphs this removal strategy can be very costly.
2. Using a DQL DELETE statement allows you to delete multiple entities of a type with a single command and without hydrating these entities. This can be very efficient to delete large object graphs from the database.
3. Using foreign key semantics onDelete="CASCADE" can force the database to remove all associated objects internally. This strategy is a bit tricky to get right but can be very powerful and fast. You should be aware however that using strategy 1 (CASCADE=REMOVE) completly by-passes any foreign key onDelete=CASCADE option, because Doctrine will fetch and remove all associated entities explicitly nevertheless.

An entity is detached from an EntityManager and thus no longer managed by invoking the EntityManager#detach($entity) method on it or by cascading the detach operation to it. Changes made to the detached entity, if any (including removal of the entity), will not be synchronized to the database after the entity has been detached.

Doctrine will not hold on to any references to a detached entity.

Example:

<?php

$em->detach($entity);

The semantics of the detach operation, applied to an entity X are as follows:

• If X is a managed entity, the detach operation causes it to become detached. The detach operation is cascaded to entities referenced by X, if the relationships from X to these other entities is mapped with cascade=DETACH or cascade=ALL (see "Transitive Persistence"). Entities which previously referenced X will continue to reference X.
• If X is a new or detached entity, it is ignored by the detach operation.
• If X is a removed entity, the detach operation is cascaded to entities referenced by X, if the relationships from X to these other entities is mapped with cascade=DETACH or cascade=ALL (see "Transitive Persistence"). Entities which previously referenced X will continue to reference X.

There are several situations in which an entity is detached automatically without invoking the detach method:

• When EntityManager#clear() is invoked, all entities that are currently managed by the EntityManager instance become detached.
• When serializing an entity. The entity retrieved upon subsequent unserialization will be detached (This is the case for all entities that are serialized and stored in some cache, i.e. when using the Query Result Cache).

The detach operation is usually not as frequently needed and used as persist and remove.

Merging entities refers to the merging of (usually detached) entities into the context of an EntityManager so that they become managed again. To merge the state of an entity into an EntityManager use the EntityManager#merge($entity) method. The state of the passed entity will be merged into a managed copy of this entity and this copy will subsequently be returned.

Example:

<?php

$detachedEntity = unserialize($serializedEntity); // some detached entity
$entity = $em->merge($detachedEntity);
// $entity now refers to the fully managed copy returned by the merge operation.
// The EntityManager $em now manages the persistence of $entity as usual.

When you want to serialize/unserialize entities you have to make all entity properties protected, never private. The reason for this is, if you serialize a class that was a proxy instance before, the private variables won't be serialized and a PHP Notice is thrown.

The semantics of the merge operation, applied to an entity X, are as follows:

• If X is a detached entity, the state of X is copied onto a pre-existing managed entity instance X' of the same identity or a new managed copy X' of X is created.
• If X is a new entity instance, an InvalidArgumentException will be thrown.
• If X is a removed entity instance, an InvalidArgumentException will be thrown.
• If X is a managed entity, it is ignored by the merge operation, however, the merge operation is cascaded to entities referenced by relationships from X if these relationships have been mapped with the cascade element value MERGE or ALL (see "Transitive Persistence").
• For all entities Y referenced by relationships from X having the cascade element value MERGE or ALL, Y is merged recursively as Y'. For all such Y referenced by X, X' is set to reference Y'. (Note that if X is managed then X is the same object as X'.)
• If X is an entity merged to X', with a reference to another entity Y, where cascade=MERGE or cascade=ALL is not specified, then navigation of the same association from X' yields a reference to a managed object Y' with the same persistent identity as Y.

The merge operation will throw an OptimisticLockException if the entity being merged uses optimistic locking through a version field and the versions of the entity being merged and the managed copy dont match. This usually means that the entity has been modified while being detached.

The merge operation is usually not as frequently needed and used as persist and remove. The most common scenario for the merge operation is to reattach entities to an EntityManager that come from some cache (and are therefore detached) and you want to modify and persist such an entity.

If you load some detached entities from a cache and you do not need to persist or delete them or otherwise make use of them without the need for persistence services there is no need to use merge. I.e. you can simply pass detached objects from a cache directly to the view.

The state of persistent entities is synchronized with the database on flush of an EntityManager which commits the underlying UnitOfWork. The synchronization involves writing any updates to persistent entities and their relationships to the database. Thereby bidirectional relationships are persisted based on the references held by the owning side of the relationship as explained in the Association Mapping chapter.

When EntityManager#flush() is called, Doctrine inspects all managed, new and removed entities and will perform the following operations.

Doctrine 2 provides the following ways, in increasing level of power and flexibility, to query for persistent objects. You should always start with the simplest one that suits your needs.