Object relations in Extbase 

A relation to one other object 

The simplest relation points from one object to a single other one — a Conference is held at a Location. In the model this is a single, typed, usually nullable property:

use TYPO3\CMS\Extbase\DomainObject\AbstractEntity;

class Conference extends AbstractEntity
{

    protected ?Location $location = null;

    protected function getLocation(): ?Location
    {
        return $this->location;
    }

}

The property is nullable because, from this object's side, the relation is optional: a conference may or may not have a location set. Reading it is a plain getter method.

In the database, the relation is stored as a single column on the owning table that holds the UID of the related record (for example a location column on the conference table). The related record does not know it is being pointed at — nothing on the Location side records the conference.

A relation to many objects 

When an object holds a collection of others, the property is an ObjectStorage — Extbase's container for related objects. A Conference collects many Comment objects, and is staffed by many Speaker objects; both are ObjectStorage properties:

<?php

namespace MyVendor\MyExtension\Domain\Model;

use TYPO3\CMS\Extbase\DomainObject\AbstractEntity;
use TYPO3\CMS\Extbase\Persistence\ObjectStorage;

class Conference extends AbstractEntity
{
    /** @var ObjectStorage<Comment> */
    protected ObjectStorage $comments;

    /** @var ObjectStorage<Speaker> */
    protected ObjectStorage $speakers;

    public function __construct()
    {
        $this->initializeObject();
    }

    public function initializeObject(): void
    {
        $this->comments = new ObjectStorage();
        $this->speakers = new ObjectStorage();
    }

    /** @return ObjectStorage<Comment> */
    public function getComments(): ObjectStorage
    {
        return $this->comments;
    }

    public function addComment(Comment $comment): void
    {
        $this->comments->attach($comment);
    }

    public function removeComment(Comment $comment): void
    {
        $this->comments->detach($comment);
    }
}

You iterate the storage to read the related objects, and add or remove single objects through dedicated addComment() / removeComment() methods that call attach() and detach(). The @var ObjectStorage<Comment> annotation is required so that IDEs and static analysis know the element type.

The two collections above look identical in PHP, but they are stored differently in the database — and that difference decides what happens when you delete the owning object.

<?php

namespace MyVendor\MyExtension\Domain\Model;

use TYPO3\CMS\Extbase\DomainObject\AbstractEntity;
use TYPO3\CMS\Extbase\Persistence\ObjectStorage;

class Conference extends AbstractEntity
{
    /**
     * @var ObjectStorage<Speaker>
     */
    protected ObjectStorage $speakers;

    public function __construct()
    {
        $this->speakers = new ObjectStorage();
    }

    public function getSpeakers(): ObjectStorage
    {
        return $this->speakers;
    }

    public function setSpeakers(ObjectStorage $speakers): void
    {
        $this->speakers = $speakers;
    }

    public function addSpeaker(Speaker $speaker): void
    {
        $this->speakers->attach($speaker);
    }

    public function removeSpeaker(Speaker $speaker): void
    {
        // Detaches the link only — the Speaker record itself is not deleted,
        // because it may still belong to other conferences.
        $this->speakers->detach($speaker);
    }
}

Behind the scenes: TCA, unidirectional and bidirectional relations 

The model property tells you that there is a relation and whether it holds one object or many. It does not, on its own, tell you the full shape of the relation. That comes from the TCA of the column: whether there is a back-reference field, an MM table, a maxitems limit, or an explicit relationship setting. Extbase reads that TCA and maps each column to one of its internal relation kinds — a single related object, a collection with a back-reference, or a collection through an MM table.

The examples above are all unidirectional: you can navigate from the conference to its location, comments and speakers, but not back. A Location usually does not know which conference is held there; a Speaker does not list the conferences they speak at. Each relation is declared once, on the owning side only.

This is why the classic cardinality labels do not apply to a single property. Terms like 1:1, 1:n, n:1 and n:m describe how both ends of a relation are constrained — and you only know both ends when the relation is bidirectional, declared on each side:

• Conference → Location on its own is a relation to one other object. Whether it is "1:1" (each location used by exactly one conference) or "n:1" (many conferences share a location) depends on a constraint that lives on the Location side — and nothing in the conference model states it.
• Likewise a relation to many becomes a "1:n" only if the child side is constrained to a single parent. As written, the comment relation simply collects children; it does not, by itself, forbid a comment from being attached elsewhere.

For everyday extension code this rarely matters, because Extbase relations are overwhelmingly unidirectional — you model the direction you actually navigate and leave the other side unaware. Reach for a bidirectional relation only when you genuinely need to navigate and query from both ends; it means declaring and maintaining the relation, and its TCA, on both models.

How Extbase loads related objects 

By default Extbase loads a relation eagerly: when the parent object is hydrated, its related objects are loaded in the same operation and the property already contains ready-to-use objects. You call $conference->getLocation() or iterate $conference->getComments() and the objects are simply there.

The order of children inside a collection is not controlled by repository queries — there is no query to override, because Extbase loads the children as part of the parent. Child ordering comes from the TCA of the relation ( foreign_sortby / foreign_default_sortby), not from $defaultOrderings or setOrderings(). This catches developers out:

Lazy loading and the N+1 query trap 

Eager loading is convenient but not free. Loading a list of 50 conferences, each with an eagerly loaded ObjectStorage of comments, means Extbase loads every comment of every conference up front — even if the list view never displays them.

Marking the relation #[Lazy] defers loading: Extbase installs a lightweight placeholder and loads the related objects only on first access. A single relation receives a \TYPO3\CMS\Extbase\Persistence\Generic\LazyLoadingProxy ; an ObjectStorage becomes a \TYPO3\CMS\Extbase\Persistence\Generic\LazyObjectStorage . Both resolve on first access.

Lazy loading solves the load-everything-up-front problem but introduces its opposite — the N+1 query problem. Consider a template that does need the relation for every row:

<f:for each="{conferences}" as="conference">
    <h2>{conference.title}</h2>
    <p>{conference.comments -> f:count()} comments</p>
</f:for>

If this were a lazy relation, it would run one query to fetch 50 conferences and then one more query per conference to load comments — 51 queries for one page. Making the relation lazy would not mean less work here; it would spread the same work across many small queries, which is usually slower than one larger one.

The rule of thumb:

• Eager (the default) when you usually need the related objects — for example a detail view that always renders the location and all comments.
• Lazy ( #[Lazy]) when you usually do not need the relation — for example a list view that shows only titles, where loading every child would be wasted work.
• If a list view does need related data for every row, neither is ideal. It is often best to step outside the ORM and fetch what the view needs in a single query.

The lazy-versus-eager decision is per relation and reversible — it is a property attribute, not a schema change — so measure with real data before optimising.

With relations and queries understood, you have the full persistence picture of an Extbase extension. For everything the ORM does not do — aggregates, bulk writes, complex joins — see When to drop out of ORM (Object Relational Mapping).