Flag content migration with Migrate module

The following example is pending the patch found at https://www.drupal.org/node/2503815

class MyCustomFlagMigration extends DrupalMigration {
  public function __construct($arguments) {
    parent::__construct($arguments);

    // requires the flag name of the source flag in query() below
    $this->sourceFlag = 'bookmarks';
    // requires the fid of the destination flag
    $this->destFlag = flag_get_flag('bookmarks');

    $this->description = t('My awesome bookmark flag migration.');

    $this->source = new MigrateSourceSQL($this->query());
    $this->destination = new MigrateDestinationFlag($this->destFlag->fid);

    // source schema from Flag-7.x-2.x
    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'fcid' => array(
          'type' => 'int',
          'not null' => TRUE,
          'description' => 'Source Flag CID',
        )
      ),
      MigrateDestinationFlag::getKeySchema()
    );

    // map fields including example source migrations Article and User
    $this->addFieldMapping('fid', $this->destFlag->fid);
    $this->addFieldMapping('entity_id', 'content_id')->sourceMigration('Article');
    $this->addFieldMapping('uid', 'uid')->sourceMigration('User');
    $this->addFieldMapping('entity_type', 'content_type');
    $this->addSimpleMappings(array('timestamp'));

  }

  public function query() {
    // query 'flag_content' for a Flag-7.x-2.x table schema
    $query = Database::getConnection('default', $this->sourceConnection)
      ->select('flag_content', 'fc')
      ->fields('fc', array(
        'fcid',
        'fid',
        'content_type',
        'content_id',
        'uid',
        'timestamp'
      ));
    
    // find flags matching this flag machine name
    $query->leftJoin('flags', 'f', 'f.fid=fc.fid');
    $query->condition('f.name', $this->sourceFlag, '=');

    return $query;
  }
}