Working with Objects

Join-table management when removing from many-to-many collections

Regarding existing rows in many-to-many join tables that refer to an entity being removed, the following applies.

When the entity being removed does not declare the many-to-many association itself (that is, the many-to-many association is unidirectional and the entity is on the inverse side), the ORM has no reasonable way to detect associations targeting the entity's class. Thus, no ORM-level handling of join-table rows is attempted and database-level constraints apply. In case of database-level ON DELETE RESTRICT constraints, the EntityManager#flush() operation may abort and a ConstraintViolationException may be thrown. No in-memory collections will be modified in this case. With ON DELETE CASCADE, the RDBMS will take care of removing rows from join tables.

When the entity being removed is part of bi-directional many-to-many association, either as the owning or inverse side, the ORM will delete rows from join tables before removing the entity itself. That means database-level ON DELETE RESTRICT constraints on join tables are not effective, since the join table rows are removed first. Removal of join table rows happens through specialized methods in entity and collection persister classes and take one query per entity and join table. In case the association uses a @JoinColumn configuration with onDelete="CASCADE", instead of using a dedicated DELETE query the database-level operation will be relied upon.

Performance of different deletion strategies

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 ORM 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) completely by-passes any foreign key onDelete=CASCADE option, because Doctrine will fetch and remove all associated entities explicitly nevertheless.

Effects of Database and UnitOfWork being Out-Of-Sync

As soon as you begin to change the state of entities, call persist or remove the contents of the UnitOfWork and the database will drive out of sync. They can only be synchronized by calling EntityManager#flush(). This section describes the effects of database and UnitOfWork being out of sync.

• Entities that are scheduled for removal can still be queried from the database. They are returned from DQL and Repository queries and are visible in collections.
• Entities that are passed to EntityManager#persist do not turn up in query results.
• Entities that have changed will not be overwritten with the state from the database. This is because the identity map will detect the construction of an already existing entity and assumes its the most up to date version.

EntityManager#flush() is never called implicitly by Doctrine. You always have to trigger it manually.

Synchronizing New and Managed Entities

The flush operation applies to a managed entity with the following semantics:

• The entity itself is synchronized to the database using a SQL UPDATE statement, only if at least one persistent field has changed.
• No SQL updates are executed if the entity did not change.

The flush operation applies to a new entity with the following semantics:

• The entity itself is synchronized to the database using a SQL INSERT statement.

For all (initialized) relationships of the new or managed entity the following semantics apply to each associated entity X:

• If X is new and persist operations are configured to cascade on the relationship, X will be persisted.
• If X is new and no persist operations are configured to cascade on the relationship, an exception will be thrown as this indicates a programming error.
• If X is removed and persist operations are configured to cascade on the relationship, an exception will be thrown as this indicates a programming error (X would be re-persisted by the cascade).
• If X is detached and persist operations are configured to cascade on the relationship, an exception will be thrown (This is semantically the same as passing X to persist()).

Synchronizing Removed Entities

The flush operation applies to a removed entity by deleting its persistent state from the database. No cascade options are relevant for removed entities on flush, the cascade remove option is already executed during EntityManager#remove($entity).

The size of a Unit of Work

The size of a Unit of Work mainly refers to the number of managed entities at a particular point in time.

The cost of flushing

How costly a flush operation is, mainly depends on two factors:

• The size of the EntityManager's current UnitOfWork.
• The configured change tracking policies

You can get the size of a UnitOfWork as follows:

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$uowSize = $em->getUnitOfWork()->size();
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The size represents the number of managed entities in the Unit of Work. This size affects the performance of flush() operations due to change tracking (see "Change Tracking Policies") and, of course, memory consumption, so you may want to check it from time to time during development.

Direct access to a Unit of Work

You can get direct access to the Unit of Work by calling EntityManager#getUnitOfWork(). This will return the UnitOfWork instance the EntityManager is currently using.

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$uow = $em->getUnitOfWork();
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Entity State

As outlined in the architecture overview an entity can be in one of four possible states: NEW, MANAGED, REMOVED, DETACHED. If you explicitly need to find out what the current state of an entity is in the context of a certain EntityManager you can ask the underlying UnitOfWork:

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<?php
switch ($em->getUnitOfWork()->getEntityState($entity)) {
    case UnitOfWork::STATE_MANAGED:
        ...
    case UnitOfWork::STATE_REMOVED:
        ...
    case UnitOfWork::STATE_DETACHED:
        ...
    case UnitOfWork::STATE_NEW:
        ...
}
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An entity is in MANAGED state if it is associated with an EntityManager and it is not REMOVED.

An entity is in REMOVED state after it has been passed to EntityManager#remove() until the next flush operation of the same EntityManager. A REMOVED entity is still associated with an EntityManager until the next flush operation.

An entity is in DETACHED state if it has persistent state and identity but is currently not associated with an EntityManager.

An entity is in NEW state if has no persistent state and identity and is not associated with an EntityManager (for example those just created via the "new" operator).

By Primary Key

The most basic way to query for a persistent object is by its identifier / primary key using the EntityManager#find($entityName, $id) method. Here is an example:

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<?php
// $em instanceof EntityManager
$user = $em->find('MyProject\Domain\User', $id);
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The return value is either the found entity instance or null if no instance could be found with the given identifier.

Essentially, EntityManager#find() is just a shortcut for the following:

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// $em instanceof EntityManager
$user = $em->getRepository('MyProject\Domain\User')->find($id);
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EntityManager#getRepository($entityName) returns a repository object which provides many ways to retrieve entities of the specified type. By default, the repository instance is of type Doctrine\ORM\EntityRepository. You can also use custom repository classes as shown later.

By Simple Conditions

To query for one or more entities based on several conditions that form a logical conjunction, use the findBy and findOneBy methods on a repository as follows:

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// $em instanceof EntityManager

// All users that are 20 years old
$users = $em->getRepository('MyProject\Domain\User')->findBy(array('age' => 20));

// All users that are 20 years old and have a surname of 'Miller'
$users = $em->getRepository('MyProject\Domain\User')->findBy(array('age' => 20, 'surname' => 'Miller'));

// A single user by its nickname
$user = $em->getRepository('MyProject\Domain\User')->findOneBy(array('nickname' => 'romanb'));
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You can also load by owning side associations through the repository:

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$number = $em->find('MyProject\Domain\Phonenumber', 1234);
$user = $em->getRepository('MyProject\Domain\User')->findOneBy(array('phone' => $number->getId()));
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The EntityRepository#findBy() method additionally accepts orderings, limit and offset as second to fourth parameters:

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$tenUsers = $em->getRepository('MyProject\Domain\User')->findBy(array('age' => 20), array('name' => 'ASC'), 10, 0);
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If you pass an array of values Doctrine will convert the query into a WHERE field IN (..) query automatically:

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$users = $em->getRepository('MyProject\Domain\User')->findBy(array('age' => array(20, 30, 40)));
// translates roughly to: SELECT * FROM users WHERE age IN (20, 30, 40)
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An EntityRepository also provides a mechanism for more concise calls through its use of __call. Thus, the following two examples are equivalent:

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// A single user by its nickname
$user = $em->getRepository('MyProject\Domain\User')->findOneBy(array('nickname' => 'romanb'));

// A single user by its nickname (__call magic)
$user = $em->getRepository('MyProject\Domain\User')->findOneByNickname('romanb');
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Additionally, you can just count the result of the provided conditions when you don't really need the data:

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// Check there is no user with nickname
$availableNickname = 0 === $em->getRepository('MyProject\Domain\User')->count(['nickname' => 'nonexistent']);
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By Criteria

The Repository implement the Doctrine\Common\Collections\Selectable interface. That means you can build Doctrine\Common\Collections\Criteria and pass them to the matching($criteria) method.

See section Filtering collections of chapter Working with Associations

By Eager Loading

Whenever you query for an entity that has persistent associations and these associations are mapped as EAGER, they will automatically be loaded together with the entity being queried and is thus immediately available to your application.

Eager Loading can also be configured at runtime through AbstractQuery::setFetchMode in DQL or Native Queries.

Eager loading for many-to-one and one-to-one associations is using either a LEFT JOIN or a second query for fetching the related entity eagerly.

Eager loading for many-to-one associations uses a second query to load the collections for several entities at the same time.

When many-to-many, one-to-one or one-to-many associations are eagerly loaded, then the global batch size configuration is used to avoid IN(?) queries with too many arguments. The default batch size is 100 and can be changed with Configuration::setEagerFetchBatchSize().

For eagerly loaded Many-To-Many associations one query has to be made for each collection.

By Lazy Loading

Whenever you have a managed entity instance at hand, you can traverse and use any associations of that entity that are configured LAZY as if they were in-memory already. Doctrine will automatically load the associated objects on demand through the concept of lazy-loading.

By DQL

The most powerful and flexible method to query for persistent objects is the Doctrine Query Language, an object query language. DQL enables you to query for persistent objects in the language of objects. DQL understands classes, fields, inheritance and associations. DQL is syntactically very similar to the familiar SQL but it is not SQL.

A DQL query is represented by an instance of the Doctrine\ORM\Query class. You create a query using EntityManager#createQuery($dql). Here is a simple example:

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// $em instanceof EntityManager

// All users with an age between 20 and 30 (inclusive).
$q = $em->createQuery("select u from MyDomain\Model\User u where u.age >= 20 and u.age <= 30");
$users = $q->getResult();
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Note that this query contains no knowledge about the relational schema, only about the object model. DQL supports positional as well as named parameters, many functions, (fetch) joins, aggregates, subqueries and much more. Detailed information about DQL and its syntax as well as the Doctrine class can be found in the dedicated chapter. For programmatically building up queries based on conditions that are only known at runtime, Doctrine provides the special Doctrine\ORM\QueryBuilder class. While this a powerful tool, it also brings more complexity to your code compared to plain DQL, so you should only use it when you need it. More information on constructing queries with a QueryBuilder can be found in Query Builder chapter.

By Native Queries

As an alternative to DQL or as a fallback for special SQL statements native queries can be used. Native queries are built by using a hand-crafted SQL query and a ResultSetMapping that describes how the SQL result set should be transformed by Doctrine. More information about native queries can be found in the dedicated chapter.

Custom Repositories

By default the EntityManager returns a default implementation of Doctrine\ORM\EntityRepository when you call EntityManager#getRepository($entityClass). You can overwrite this behaviour by specifying the class name of your own Entity Repository in the Attribute or XML metadata. In large applications that require lots of specialized DQL queries using a custom repository is one recommended way of grouping these queries in a central location.

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<?php
namespace MyDomain\Model;

use MyDomain\Model\UserRepository;
use Doctrine\ORM\EntityRepository;
use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity(repositoryClass: UserRepository::class)]
class User
{

}

class UserRepository extends EntityRepository
{
    /** @return Collection<User> */
    public function getAllAdminUsers(): Collection
    {
        return $this->_em->createQuery('SELECT u FROM MyDomain\Model\User u WHERE u.status = "admin"')
                         ->getResult();
    }
}
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You can access your repository now by calling:

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<?php
// $em instanceof EntityManager

$admins = $em->getRepository('MyDomain\Model\User')->getAllAdminUsers();
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