django-db-constraints
What is this?
Add database table-level constraints to your Django model's Meta class and have makemigrations add the appropriate migration.
class Foo(models.Model):
bar = models.IntegerField()
baz = models.IntegerField()
class Meta:
db_constraints = {
'bar_equal_baz': 'check (bar = baz)',
}This should generate a migration like so:
class Migration(migrations.Migration):
initial = True
dependencies = [
]
operations = [
migrations.CreateModel(
name='Foo',
fields=[
('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
('bar', models.IntegerField()),
('baz', models.IntegerField()),
],
),
django_db_constraints.operations.AlterConstraints(
name='Foo',
db_constraints={'bar_equal_baz': 'check (bar = baz)'},
),
]The resulting SQL applied:
CREATE TABLE "sample_foo" ("id" serial NOT NULL PRIMARY KEY, "bar" integer NOT NULL, "baz" integer NOT NULL)
ALTER TABLE "sample_foo" ADD CONSTRAINT "bar_equal_baz" check (bar = baz)Composite foreign keys
It's possible to support composite foreign keys if you have a unique key on your reference model:
(Why are composite foreign keys useful?)
class Bar(models.Model):
baz = models.IntegerField()
class Meta:
unique_together = ('id', 'baz')
class Foo(models.Model):
bar = models.ForeignKey(Bar)
baz = models.IntegerField()
class Meta:
db_constraints = {
'composite_fk': 'foreign key (bar_id, baz) references sample_bar (id, baz)',
}Results in:
ALTER TABLE "sample_foo" ADD CONSTRAINT "composite_fk" foreign key (bar_id, baz) references sample_bar (id, baz)Notes
Migration operation ordering
Given that nothing will depend on a constraint operation, they're simply added to the end of the list of operations for a migration. This includes operations that drop fields used in a constraint as the database drop will any related constraints as well (at least with PostgreSQL).
Caveats
It's possible to end up in a situation where the constraints are declared on the Meta class but do not exist in the database due to a database dropping a constraint implicitly when a field in the constraint is dropped.
Please test your constraints!
I encourage folks to write tests for their constraints to ensure they write are actually applied in the database.
Acknowledgements
Thanks to @schinckel and @MarkusH for their advice and ideas.
Installation
pip install django-db-constraints
in your settings.py:
INSTALLED_APPS = [
'django_db_constraints',
…
]