The list of fields to extract from the `Request` object and use when querying the database. This can either be a simple array of field names, or a set of key/value pairs, which map the field names in the request to database field names.

For example, if you had a form field name `username`, which mapped to a database field named username, you could use the following in the `'fields'` configuration: {{{ 'fields' => array('username' => 'name', 'group'), }}} This is especially relevant for document databases, where you may want to map a form field to a nested document field: {{{ 'fields' => array('username' => 'login.username', 'password'), }}}