Where does the value of entity come from in a field of a content type?

Question

Where does the value of entity come from in a field of a content type?

I am trying to understand how the property (if it is a property) entity is related to the field.

This question is more informative, I describe my case below.

I have a type of content News, in this I added field of type image called "field_image" with a limit of a single value.

Type of content: News Field: field_image

Debuggin:

I programmatically load the node:

// Return Drupal\node\Entity\Node;
$node = $this->entityTypeManager->getStorage("node")->load(1);

// {\Drupal\file\Plugin\Field\FieldType\FileFieldItemList}
kint($node->field_image);

// {Drupal\file\Entity\File}
kint($node->field_image->entity); // from where comes from entity?

From the last example I show above ($node->field_image->entity), where does the property entity appear, how can field_image access it and know that it is a file type field?

I'm just trying to understand these internal Drupal relationships. Thank you very much for helping me understand this. regards

Answer 1

It's literally magic (well, PHP magic methods anyway).

FieldItemBase implements __get(), which will respond to any property on the object (existing or not):

public function __get($name) {
  // There is either a property object or a plain value - possibly for a
  // not-defined property. If we have a plain value, directly return it.
  if (isset($this->properties[$name])) {
    return $this->properties[$name]->getValue();
  }
  elseif (isset($this->values[$name])) {
    return $this->values[$name];
  }
}

And the image field type extends the entity reference field type, which maintains an entity property (see EntityReferenceItem::propertyDefinitions).

So when you request the non-existent entity property on the field class, FieldItemBase hooks it up for you via the $properties array, and you get an entity back.