code: public function authenticate()
      {
          $this->_authenticateSetup();
  
          $dbSelect   = $this->_authenticateCreateSelect();
          $identity   = $this->_authenticateQuerySelect($dbSelect);
          $authResult = $this->_authenticateValidateResultSet($identity);
  
          if ($authResult instanceof Zend_Auth_Result) {
              return $authResult;
          }
  
          /// _authenticateValidateResult() attempts to make certain that only
          /// one record was returned in the resultset.
          return $this->_authenticateValidateResult(array_shift($identity));
      }
docs: Called to attempt an authentication.
  
  Previous to this call, this adapter would have already been configured
  with all necessary information to successfully connect to a database
  table and attempt to find a record matching the provided identity.
  
  @return Zend_Auth_Result
  @see http://framework.zend.com/manual/1.12/en/zend.auth.introduction.html#zend.auth.introduction.results Zend_Auth_Result
  @throws Zend_Auth_Adapter_Exception if answering the authentication
  query is impossible