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Drupal 8 Entity Validation and Typed Data Explained

Data validation is a very important part of any application. Drupal 7 has a great Form API that can handle complex validation of submitted data, which can then be turned into entities. However, form level validation is problematic. For example, it becomes difficult to handle entity (data) validation programmatically. We have to either reimplement validation logic or mimic form submissions in code. This makes any data interaction dependent on the form system and this is a bad idea.

Drupal 8 logo

With the introduction of subsystems such as the REST API, Drupal 8 needed something better to handle this problem. The Symfony Validation component was chosen and Drupal 8 builds on top of it to tailor for the realties of the Typed Data and plugin based Entity system. So now, form submissions validate entities just like REST calls do or any other programmatic interaction with the entities easily can.

In this article, and its followup, we will explore the Drupal 8 Entity Validation API, see how it works, and how it can be extended. To better understand this, we will also take a look at the Typed Data API which underpins the entity system in Drupal 8. There will be some code examples that already exist in core but we will also write a bit of our own code which can be found in this git repository within the demo module.

Typed Data

Typed Data is the Drupal 8 API that was built to provide a consistent way of interacting with data or metadata about the data itself. Why is this important for our topic? Because validation is defined and invoked on typed data objects.

Two important components of this API stand out: the data definition and the DataType plugins. The role of the former is to define data and how interaction works with it (including things like settings or validation constraints). The role of the latter is to provide a way to get and set values from that type of data. When they are instantiated, data type plugins make use of data definition instances passed on by the plugin manager. The latter can also infer which data definition needs to be used by a DataType plugin type.

Let’s see an example:

$definition = DataDefinition::create('string')
    ->addConstraint('Length', array('max' => 20));

We created a string data definition and applied the Length constraint to it. Constraints are a key part of the validation API and they define the type of validation that will run on the data. They are coupled with a validator that actually performs the task, but we will see more about constraints and validators in the second part of this series.

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