Function using use_db

    /**
     * Retrieve a user by their unique identifier.
     * 
     * @param  mixed $identifier
     * @return \Illuminate\Auth\UserInterface|null
     */
    public function retrieveById($identifier)
    {
        if ($entries = $this->searchLdap($identifier)) {
            if (Config::get('l4-openldap::use_db')) {
                $ldap_value = $entries[0][Config::get('l4-openldap::ldap_field')][0];
                $user = $this->db_conn->table(Config::get('l4-openldap::db_table'))->where(Config::get('l4-openldap::db_field'), '=', $ldap_value)->first();
                if (Config::get('l4-openldap::eloquent')) {
                    return $this->createModel()->newQuery()->find($user->id);
                } else {
                    return new GenericUser(get_object_vars($user));
                }
            } else {
                return $this->createGenericUserFromLdap($entries[0]);
            }
        }        
    }