How to define two fields "unique" as couple

Question

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.

Answer 1

There is a simple solution for you called unique_together which does exactly what you want.

For example:

class MyModel(models.Model):   field1 = models.CharField(max_length=50)   field2 = models.CharField(max_length=50)    class Meta:     unique_together = ('field1', 'field2',) 

And in your case:

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',) 

Answer 2

Django 2.2+

Using the constraints features UniqueConstraint is preferred over unique_together.

From the Django documentation for unique_together:

Use UniqueConstraint with the constraints option instead.
UniqueConstraint provides more functionality than unique_together.
unique_together may be deprecated in the future.

For example:

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:         constraints = [             models.UniqueConstraint(fields=['journal_id', 'volume_number'], name='name of constraint')         ]