<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def validate(self, value):
 """ Returns a cleaned and validated value. Raises a ValidationError if there's a problem """

        if value is None:
            if self.required:
                raise ValidationError('{} - None values are not allowed'.format(self.column_name or self.db_field))
        return value