access data from a secondary database

This example data provider allows your resource to draw data from a secondary database (i.e. the kind Drupal defines in settings.php's $databases array.) This data provider refers to a dbName value, which is pulled from the resource's annotation data, and which describes which database Drupal should use.

It may be placed anywhere in a PSR-4 filesystem path, but in the given code, it exists at my_module/src/Plugin/DataProvider/DataProviderDbQueryExternalDB.php.

<?php

/**
 * @file
 * Contains \Drupal\my_module\Plugin\DataProvider\DataProviderDbQueryExternalDB.
 */

namespace Drupal\my_module\Plugin\DataProvider;

use Drupal\restful\Plugin\resource\DataProvider\DataProviderDbQuery;

class DataProviderDbQueryExternalDB extends DataProviderDbQuery {
  /**
   * Get a basic query object, using the specified database.
   *
   * @return \SelectQuery
   *   A new SelectQuery object for this connection.
   */
  protected function getQuery() {
    $db_name = $this->options['dbName'];
    $table = $this->getTableName();

    return \Database::getConnection('default', $db_name)->select($table)->fields($table);
  }

}

This would be used by a resource as follows:

?php

/**
 * @file
 * Contains \Drupal\my_module\Plugin\resource\db_query\participant\Myresource__1_0.
 */

namespace Drupal\my_module\Plugin\resource\db_query\myresource;

use Drupal\restful\Plugin\resource\ResourceDbQuery;
use Drupal\restful\Plugin\resource\ResourceInterface;

/**
 * Class Myresource__1_0
 * @package Drupal\my_module\Plugin\resource
 *
 * @Resource(
 *   name = "myresource:1.0",
 *   resource = "myresource",
 *   label = "My Resource",
 *   description = "Expose my resource to the REST API.",
 *   authenticationTypes = TRUE,
 *   authenticationOptional = TRUE,
 *   dataProvider = {
 *     "dbName": "my_secondary_db", // As per the key specified in settings.php 
 *     "tableName": "my_table",
 *     "idColumn": "my_tables_primary_key_column",
 *     "primary": "my_tables_primary_key_column",
 *     "idField": "my_tables_primary_key_column",
 *   },
 *   majorVersion = 1,
 *   minorVersion = 0
 * )
 */
class Myresource__1_0 extends ResourceDbQuery implements ResourceInterface {

  // ...your method overrides here...

  /**
   * {@inheritdoc}
   */
  protected function dataProviderClassName() {
    return '\Drupal\my_module\Plugin\DataProvider\DataProviderDbQueryExternalDB';
  }
}