class Volume(models.Model):
    id = models.AutoField(primary_key=True)
journal_id = models.ForeignKey(Journals, db_column='jid', null=True,
    verbose_name='Journal')
volume_number = models.CharField('Volume Number', max_length=100)
comments = models.TextField('Comments', max_length=4000, blank=True)


class Meta:
    unique_together = 'journal_id', 'volume_number'
How to define two fields "unique" as couple