def find_field(self, field=None, alias=None):
        """
        Finds a field by name or alias.

        :param field: string of the field name or alias, dict of {'alias': field}, or a Field instance
        :type field: str or dict or Field

        :returns: The field if it is found, otherwise None
        :rtype: :class:`Field <querybuilder.fields.Field>` or None
        """
        if alias:
            field = alias
        field = FieldFactory(field, table=self, alias=alias)
        identifier = field.get_identifier()
        for field in self.fields:
            if field.get_identifier() == identifier:
                return field
        return None