code: private function valueExists($targetClass, $query, $value)
      {
          $db = $targetClass::getDb();
          $exists = false;
  
          if ($this->forceMasterDb && method_exists($db, 'useMaster')) {
              $db->useMaster(function ($db) use ($query, $value, &$exists) {
                  $exists = $this->queryValueExists($query, $value);
              });
          } else {
              $exists = $this->queryValueExists($query, $value);
          }
  
          return $exists;
      }
docs: Check whether value exists in target table
  
  @param string $targetClass
  @param QueryInterface $query
  @param mixed $value the value want to be checked
  @return bool