/**
  * Returns a boolean query param based on the current WP_Query
  * @param array $query_vars
  * @return string The resulting boolean query parameter
  */
 public function wpToBooleanQuery($query_vars)
 {
     $meta_query = new WP_Meta_Query();
     $meta_query->parse_query_vars($query_vars);
     if (count($meta_query->queries) > 0) {
         $expressionSet = new Lift_Expression_Set(strtolower($meta_query->relation));
         foreach ($meta_query->queries as $subquery) {
             if ($subquery['key'] == $this->meta_key) {
                 if ($subquery['compare'] === 'LIKE') {
                     foreach ((array) $subquery['value'] as $value) {
                         $expressionSet->addExpression(new Lift_Expression_Field($this->name, $value));
                     }
                 } elseif ($subquery['compare'] === 'NOT LIKE') {
                     $subExpression = new Lift_Expression_Set('NOT');
                     foreach ((array) $subquery['value'] as $value) {
                         $subExpression->addExpression(new Lift_Expression_Field($this->name, $value));
                     }
                     $expressionSet->addExpression($subExpression);
                 }
             }
         }
         return $expressionSet;
     }
     return '';
 }
 protected function push()
 {
     if ($this->buffer_start !== null) {
         // extract string from buffer start to current position
         $buffer = substr($this->string, $this->buffer_start, $this->position - $this->buffer_start);
         // clean buffer
         $this->buffer_start = null;
         // throw token into current scope
         if (is_null($this->currentSet)) {
             if (in_array($buffer, array('and', 'or', 'not'))) {
                 $this->currentSet = new Lift_Expression_Set($buffer);
             } else {
                 list($name, $value) = explode(':', $buffer);
                 $is_str = false;
                 if ($value[0] === "'") {
                     $is_str = true;
                     $value = substr($value, 1, -1);
                 }
                 $this->currentSet = new Lift_Expression_Field($name, $value, $is_str);
             }
         } else {
             list($name, $value) = explode(':', $buffer);
             $is_str = false;
             if ($value[0] === "'") {
                 $is_str = true;
                 $value = substr($value, 1, -1);
             }
             $exp = new Lift_Expression_Field($name, $value, $is_str);
             $this->currentSet->addExpression($exp);
         }
     }
 }