Important: #100207 - Let DataMapper::createEmptyObject() use doctrine/instantiator¶

Introduction¶

This document explains the intended way in which the Extbase ORM thaws/hydrates objects.

Hydrating objects¶

Hydrating (the term originates from doctrine/orm), or in Extbase terms thawing, is the act of creating an object from a given database row. The responsible class involved is the DataMapper. During the process of hydrating, the DataMapper creates objects to map the raw database data onto.

Before diving into the framework internals, let's take a look at models from the user's perspective.

Creating objects with constructor arguments¶

Imagine you have a table tx_extension_domain_model_blog and a corresponding model or entity (entity is used as a synonym here) Vendor\Extension\Domain\Model\Blog.

Now, also imagine there is a domain rule which states, that all blogs must have a title. This rule can easily be followed by letting the blog class have a constructor with a required argument string $title.

class Blog extends AbstractEntity
{
    protected ObjectStorage $posts;

    public function __construct(protected string $title)
    {
        $this->posts = new ObjectStorage();
    }
}

This example also shows how the posts property is initialized. It is done in the constructor because PHP does not allow setting a default value that is of type object.

Hydrating objects with constructor arguments¶

Whenever the user creates new blog objects in extension code, the aforementioned domain rule is followed. It is also possible to work on the posts ObjectStorage without further initialization. new Blog('title') is all I need to create a blog object with a valid state.

What happens in the DataMapper however, is a totally different thing. When hydrating an object, the DataMapper cannot follow any domain rules. Its only job is to map the raw database values onto a Blog instance. The DataMapper could of course detect constructor arguments and try to guess which argument corresponds to what property but only if there is an easy mapping, i.e. if the constructor takes argument string $title and updates property title with it.

To avoid possible errors due to guessing, the DataMapper simply ignores the constructor at all. It does so with the help of the library doctrine/instantiator.

This pretty much explains the title of this document in detail. But there is more to all this.

Initializing objects¶

Have a look at the $posts property in the example above. If the DataMapper ignores the constructor, that property is in an invalid state, i.e. uninitialized.

To address this problem and possible others, the DataMapper will call the method initializeObject(): void on models, if it exists.

Here is an updated version of the model:

class Blog extends AbstractEntity
{
    protected ObjectStorage $posts;

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

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

This example demonstrates how Extbase expects the user to set up their model(s). If method initializeObject() is used for initialization logic that needs to be triggered on initial creation AND on hydration. Please mind that __construct() SHOULD call initializeObject().

If there are no domain rules to follow, the recommended way to set up a model would then still be to define a __construct() and initializeObject() method like this:

class Blog extends AbstractEntity
{
    protected ObjectStorage $posts;

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

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

Mutating objects¶

I'd like to add a few more words on mutators (setter, adder, etc.). One might think that DataMapper uses mutators during object hydration but it DOES NOT. mutators are the only way for the user (developer) to implement business rules besides using the constructor.

The DataMapper uses the @internal method AbstractDomainObject::_setProperty() to update object properties. This looks a bit dirty and is a way around all business rules but that's what the DataMapper needs in order to leave the mutators to the users.

Property visibility¶

One important thing to know is that Extbase needs entity properties to be protected or public. As written in the former paragraph, AbstractDomainObject::_setProperty() is used to bypass setters. AbstractDomainObject however, is not able to access private properties of child classes, hence the need to have protected or public properties.

Dependency injection¶

Without digging too deep into this topic the following statements have to be made. Extbase expects entities to be so called prototypes, i.e. classes that do have a different state per instance. DataMapper DOES NOT use dependency injection for the creation of entities, i.e. it does not query the object container. This also means, that dependency injection is not possible in entities.

If you think that your entities need to use/access services, you need to find other ways to implement it.