title: How to define two fields "unique" as couple
body: <p>Is there a way to define a couple of fields as unique in Django?</p>
  
  <p>I have a table of volumes (of journals) and I don't want more then one volume number for the same journal.</p>
  
  <pre><code>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)
  </code></pre>
  
  <p>I tried to put <code>unique = True</code> as attribute in the fields <code>journal_id</code> and <code>volume_number</code> but it doesn't work.</p>