Is there any way to alter the response of JSON API module?

Question

I am using JSON API module to expose data using REST API. I want to alter its response. I am trying to altering it using hook_ENTITY_TYPE_load. If I change any existing property it shows the updated value. But if I try to add custom property it doesn't show the custom property in response. The two hooks I used are below:

function my_custom_entity_type_alter(array &$entity_types) {
  $entity_types['node']->set('flag_count', 10);
}

function my_custom_node_load($nodes) {
  foreach ($nodes as &$node) {
    $node->flag_count = 10;
  }
}

I need to know if there is any possible way to alter the json api response with custom properties?

Answer 1

Please see how jsonapi_entity_base_field_info adds a computed property to the file entities. That should do.

If you don't want to create a virtual field, you can add a normalizer that only acts for that specific content type as a service. You'll need to use a higher priority than serializer.normalizer.entity.jsonapi (set to 21). Your custom normalizer can inherit from Drupal\jsonapi\Normalizer\ContentEntityNormalizer and add your custom stuff in getFields.

Answer 2

You can create your own custom normalizers to alter the JSON response.

Some good resources:

• https://www.drupal.org/docs/8/api/serialization-api/how-the-serializer-works - An overview of how serializers work with normalizing.
• https://www.mediacurrent.com/blog/using-normalizers-alter-rest-json-structure-drupal-8 - A good tutorial on using normalizers to alter REST JSON structure.

Answer 3

I suppose you should be using hook_entity_type_build rather than hook_entity_type_alter, which states:

Do not use this hook to add information to entity types, unless one of the following is true:

• You are filling in default values.
• You need to dynamically add information only in certain circumstances.
• Your hook needs to run after hook_entity_type_build() implementations.

Use hook_entity_type_build() instead in all other cases.

And EntityTypeInterface states

Additional information can be provided by modules: hook_entity_type_build() can be implemented to define new properties, while hook_entity_type_alter() can be implemented to alter existing data and fill-in defaults.

You might want to try something like this instead:

function my_custom_entity_type_build(array &$entity_types) {
  /** @var $entity_types \Drupal\Core\Entity\EntityTypeInterface[] */
  $entity_types['node']
    ->set('flag_count', 10);
}