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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:

1<?php

namespace Documents;

/** @MappedSuperclass */
abstract class BaseDocument
{
}


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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:

1<?php

namespace Documents;

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

/**
 * @Document
 */
class Employee extends Person
{
    // ...
}


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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:

1<?php

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

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


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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:

1<?php

namespace Documents;

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

/**
 * @Document
 */
class Employee extends Person
{
    // ...
}


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Collection Per Class Inheritance

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

1<?php

namespace Documents;

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

/**
 * @Document
 */
class Employee extends Person
{
    // ...
}


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