entity_reference.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the Entity Reference
 * module.
 */

/**
 * Implements hook_field_schema().
 */
function entity_reference_field_schema($field) {
  $schema = array(
    'columns' => array(
      'target_id' => array(
        'description' => 'The ID of the target entity.',
        'type' => 'int',
        'unsigned' => TRUE,
        'not null' => TRUE,
      ),
      'revision_id' => array(
        'description' => 'The revision ID of the target entity.',
        'type' => 'int',
        'unsigned' => TRUE,
        'not null' => FALSE,
      ),
    ),
    'indexes' => array(
      'target_id' => array(
        'target_id',
      ),
    ),
  );

  // Create a foreign key to the target entity type base type.
  $entity_manager = Drupal::service('plugin.manager.entity');
  if (is_subclass_of($entity_manager
    ->getControllerClass($field['settings']['target_type'], 'storage'), 'Drupal\\Core\\Entity\\DatabaseStorageController')) {
    $entity_info = $entity_manager
      ->getDefinition($field['settings']['target_type']);
    $base_table = $entity_info['base_table'];
    $id_column = $entity_info['entity_keys']['id'];
    $schema['foreign keys'][$base_table] = array(
      'table' => $base_table,
      'columns' => array(
        'target_id' => $id_column,
      ),
    );
  }
  return $schema;
}