Extract query pattern from operations.

def pattern(self):
        """Extract query pattern from operations."""
        if not self._pattern:

            # trigger evaluation of operation
            if (self.operation in ['query', 'getmore', 'update', 'remove'] or
                    self.command in ['count', 'findandmodify']):
                self._pattern = self._find_pattern('query: ')
            elif self.command == 'find':
                self._pattern = self._find_pattern('filter: ')

        return self._pattern