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Inheritance Mapping

Doctrine currently offers two supported methods of inheritance: single collection and collection per class inheritance.

Mapped Superclasses

A mapped superclass is an abstract or concrete class that provides mapping information for its subclasses, but is not itself a document. Typically, the purpose of such a mapped superclass is to define state and mapping information that is common to multiple document classes.

Just like non-mapped classes, mapped superclasses may appear in the middle of an otherwise mapped inheritance hierarchy (through single collection or collection per class) inheritance.

A mapped superclass cannot be a document and is not queryable.

Example:

<?php

namespace Documents;

/** @MappedSuperclass */
abstract class BaseDocument
{
}
<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <mapped-superclass name="Documents\BaseDocument">
  </mapped-superclass>
</doctrine-mongo-mapping>
Documents\BaseDocument:
    type: mappedSuperclass

Single Collection Inheritance

In single collection inheritance, each document is stored in a single collection and a discriminator field is used to distinguish one document type from another.

Simple example:

<?php

namespace Documents;

/**
 * @Document
 * @InheritanceType("SINGLE_COLLECTION")
 * @DiscriminatorField("type")
 * @DiscriminatorMap({"person"="Person", "employee"="Employee"})
 */
class Person
{
    // ...
}

/**
 * @Document
 */
class Employee extends Person
{
    // ...
}
<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <document name="Documents\Person" inheritance-type="SINGLE_COLLECTION">
    <discriminator-field name="type" />
    <discriminator-map>
        <discriminator-mapping value="person" class="Person" />
        <discriminator-mapping value="employee" class="Employee" />
    </discriminator-map>
  </document>
</doctrine-mongo-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <document name="Documents\Employee">
  </document>
</doctrine-mongo-mapping>
Documents\Person:
  type: document
  inheritanceType: SINGLE_COLLECTION
  discriminatorField: type
  discriminatorMap:
    person: Person
    employee: Employee

The discriminator value allows Doctrine to infer the class name to instantiate when hydrating a document. If a discriminator map is used, the discriminator value will be used to look up the class name in the map.

Now, if we query for a Person and its discriminator value is employee, we would get an Employee instance back:

<?php

$employee = new Employee();
// ...
$dm->persist($employee);
$dm->flush();

$employee = $dm->find('Person', $employee->getId()); // instanceof Employee

Even though we queried for a Person, Doctrine will know to return an Employee instance because of the discriminator map!

If your document structure has changed and you've added discriminators after already having a bunch of documents, you can specify a default value for the discriminator field:

<?php

namespace Documents;

/**
 * @Document
 * @InheritanceType("SINGLE_COLLECTION")
 * @DiscriminatorField("type")
 * @DiscriminatorMap({"person"="Person", "employee"="Employee"})
 * @DefaultDiscriminatorValue("person")
 */
class Person
{
    // ...
}

/**
 * @Document
 */
class Employee extends Person
{
    // ...
}
<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <document name="Documents\Person" inheritance-type="SINGLE_COLLECTION">
    <discriminator-field name="type" />
    <discriminator-map>
        <discriminator-mapping value="person" class="Person" />
        <discriminator-mapping value="employee" class="Employee" />
    </discriminator-map>
    <default-discriminator-value value="person" />
  </document>
</doctrine-mongo-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <document name="Documents\Employee">
  </document>
</doctrine-mongo-mapping>
Documents\Person:
  type: document
  inheritanceType: SINGLE_COLLECTION
  discriminatorField: type
  defaultDiscriminatorValue: person
  discriminatorMap:
    person: Person
    employee: Employee

Collection Per Class Inheritance

With collection per class inheritance, each document is stored in its own collection and contains all inherited fields:

<?php

namespace Documents;

/**
 * @Document
 * @InheritanceType("COLLECTION_PER_CLASS")
 */
class Person
{
    // ...
}

/**
 * @Document
 */
class Employee extends Person
{
    // ...
}
<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <document name="Documents\Person" inheritance-type="COLLECTION_PER_CLASS">
  </document>
</doctrine-mongo-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mongo-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">
  <document name="Documents\Employee">
  </document>
</doctrine-mongo-mapping>
Documents\Person:
  type: document
  inheritanceType: COLLECTION_PER_CLASS

A discriminator is not needed with this type of inheritance since the data is separated in different collections.