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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.

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5 Answers 5

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964

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',)
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11 Comments

I would say you'll get a "ValidationError" exception. Have a look at the Django docs: Model.validate_unique
How would you handle this say if volume_number could be null? Mysql won't seem to enforce unique in that case.
FYI it throws a django.db.utils.IntegrityError if you try to add a duplicate.
@Greg - According to the ANSI standard SQL:2003 (and previous ones as well), a UNIQUE constraint should disallow duplicate non-NULL values, but allow multiple NULL values (see draft wiscorp.com/sql_2003_standard.zip, Framework, p. 22). If you want your unique constraint to disallow multiple null values, you are probably doing something wrong, like using NULL as a meaningfull value. Remember, nullable field says "We don't always have a value for that field but when we do it must be unique.".
What about multiple unique_together constraints? For example - when I want to have mode columns to be unique in the scope of the parent? Well, this property is actually a tuple itself, see: docs.djangoproject.com/en/1.4/ref/models/options/… So your constraint should be more explicitly written as: unique_together = (('journal_id', 'volume_number',),).
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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') ]
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4 Comments

In what situation would the 'name' parameter of the UniqueConstraint be used for? I assume it works like the name parameter of a URL path?
@user7733611 naming the constraint can be helpful in a number of situations. For example if you are connecting to a legacy database, or if you just want the constraint names to be more human readable in the database. One time I migrated the character set of a MySQL database and Django's generated constraint names were actually too long for our particular target.
Not 100% sure it comes from UniqueConstraint but I get weird psycopg2.errors.DuplicateTable: relation "name_of_the_constraint" already exists when I switch to Postgres
It should be noted that text fields like CharField can be case sensitive or case insensitive depending on your database configurations!
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in Django 4.0,

The new *expressions positional argument of UniqueConstraint() enables creating functional unique constraints on expressions and database functions. For example:

from django.db import models from django.db.models import UniqueConstraint from django.db.models.functions import Lower class MyModel(models.Model): first_name = models.CharField(max_length=255) last_name = models.CharField(max_length=255) class Meta: constraints = [ UniqueConstraint( Lower('first_name'), Lower('last_name').desc(), name='first_last_name_unique', ), ]
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2 Comments

As of Django 4.1, you can now specify a validation_error_message argument in your constraint function.
It's actually violation_error_message in case anybody is looking ... docs.djangoproject.com/en/4.1/ref/models/constraints/…
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Yes you can define more than one field as unique using Django class Meta as this 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: unique_together = ('volume_number', 'journal_id')

Note:
To make things go writes you should not add attribute unique=True to any field that you define in unique_together attribute otherwise it will not work as unique together.

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1 Comment

As noted by other answers, this method is now deprecated in favor of using constraints such as UniqueConstraint.
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In Django 4.1.1

from django.db import models class Volume(models.Model): field_1 = models.CharField("field 1", max_length=100,) field_2 = models.CharField("field 2", max_length=100,) field_3 = models.CharField("field 3", max_length=100,) class Meta: unique_together = [['field_1', 'field_2']]
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1 Comment

For convenience, unique_together can be a single list when dealing with a single set of fields: unique_together = ['field_1', 'field_2']

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