How to define two fields “unique” as couple

Is there a way to define a couple of fields as unique in Django?

I have a table of volumes (of journals) and I don’t want more then one volume number for the same journal.

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)

I tried to put unique = True as attribute in the fields journal_id and volume_number but it doesn’t work.