Drupal 8 || 9 - Getting Entity Reference ID from GraphQL Field Resolvers

drupal_9_getting_entity_reference_id_in_graphql.md

$registry->addFieldResolver('ContentType', 'graphqlField',
 $builder->compose(
   $builder->produce('entity_id')
     ->map('entity', $builder->fromParent()),
   $builder->callback(function ($id) {
     // First: get node by id.
     $node = \Drupal::entityTypeManager()->getStorage('node')->load($id);
     // Second: get entity reference id.
     $entity_id = $node->get('field_entity_reference_name')->first()->getValue()['target_id'];
     
     return $entity_id;
   })
 )
);