Continuing on our previous example let's add a new field that is an entity reference for a taxonomy term.

We again need to do similar steps to add the field to the schema and add its resolver.

Add the schema declaration

The first step as seen in the introduction is to add the types and fields in the schema. We can do this directly in the schema string in your own schema implementation (src/Plugin/GraphQL/Schema/SdlSchemaMyDrupalGql.php).

...
​
type Article implements NodeInterface {
    id: Int!
    title: String!
    creator: String
    tags: [TagTerm]
}
​
type TagTerm {
    id: Int
    name: String
}
​
...

Now we have an article that also has a custom entity reference field to a taxonomy term (the field name in Drupal is field_tags) and we make a new type TagTerm that has the necessary information about this term.

We will need to resolve not only the tags field but also the id and name of the term.

Adding resolvers

Again inside the getResolverRegistry method :

/**
 * {@inheritdoc}
 */
protected function getResolverRegistry() {
​
  ...
​
  $registry->addFieldResolver('Article', 'tags',
    $builder->produce('entity_reference')
      ->map('entity', $builder->fromParent())
      ->map('field', $builder->fromValue('field_tags'))
  );
​
  $registry->addFieldResolver('TagTerm', 'id',
    $builder->produce('entity_id')
      ->map('entity', $builder->fromParent())
  );
​
  $registry->addFieldResolver('TagTerm', 'name',
    $builder->produce('entity_label')
      ->map('entity', $builder->fromParent())
  );
​
  ...
​
  return $registry;
}

Notice how again we are using common data producers provided by the module like entity_id, entity_label and also entity_reference in this case.