In Django, each model has a primary key. By default, this primary key consists\nof a single field.
\nIn most cases, a single primary key should suffice. In database design,\nhowever, defining a primary key consisting of multiple fields is sometimes\nnecessary.
\nTo use a composite primary key, when defining a model set the pk attribute\nto be a CompositePrimaryKey:
class Product(models.Model):\n name = models.CharField(max_length=100)\n\n\nclass Order(models.Model):\n reference = models.CharField(max_length=20, primary_key=True)\n\n\nclass OrderLineItem(models.Model):\n pk = models.CompositePrimaryKey("product_id", "order_id")\n product = models.ForeignKey(Product, on_delete=models.CASCADE)\n order = models.ForeignKey(Order, on_delete=models.CASCADE)\n quantity = models.IntegerField()\nThis will instruct Django to create a composite primary key\n(PRIMARY KEY (product_id, order_id)) when creating the table.
A composite primary key is represented by a tuple:
>>> product = Product.objects.create(name="apple")\n>>> order = Order.objects.create(reference="A755H")\n>>> item = OrderLineItem.objects.create(product=product, order=order, quantity=1)\n>>> item.pk\n(1, "A755H")\nYou can assign a tuple to the pk attribute.\nThis sets the associated field values:
>>> item = OrderLineItem(pk=(2, "B142C"))\n>>> item.pk\n(2, "B142C")\n>>> item.product_id\n2\n>>> item.order_id\n"B142C"\nA composite primary key can also be filtered by a tuple:
>>> OrderLineItem.objects.filter(pk=(1, "A755H")).count()\n1\nWe’re still working on composite primary key support for\nrelational fields, including\nGenericForeignKey fields, and the Django admin. Models with composite\nprimary keys cannot be registered in the Django admin at this time. You can\nexpect to see this in future releases.
Django doesn’t support migrating to, or from, a composite primary key after the\ntable is created. It also doesn’t support adding or removing fields from the\ncomposite primary key.
\nIf you would like to migrate an existing table from a single primary key to a\ncomposite primary key, follow your database backend’s instructions to do so.
\nOnce the composite primary key is in place, add the CompositePrimaryKey\nfield to your model. This allows Django to recognize and handle the composite\nprimary key appropriately.
While migration operations (e.g. AddField, AlterField) on primary key\nfields are not supported, makemigrations will still detect changes.
In order to avoid errors, it’s recommended to apply such migrations with\n--fake.
Alternatively, SeparateDatabaseAndState may be used to execute the\nbackend-specific migrations and Django-generated migrations in a single\noperation.
Relationship fields, including\ngeneric relations do not support composite primary\nkeys.
\nFor example, given the OrderLineItem model, the following is not\nsupported:
class Foo(models.Model):\n item = models.ForeignKey(OrderLineItem, on_delete=models.CASCADE)\nBecause ForeignKey currently cannot reference models with composite primary\nkeys.
To work around this limitation, ForeignObject can be used as an\nalternative:
class Foo(models.Model):\n item_order_id = models.CharField(max_length=20)\n item_product_id = models.IntegerField()\n item = models.ForeignObject(\n OrderLineItem,\n on_delete=models.CASCADE,\n from_fields=("item_order_id", "item_product_id"),\n to_fields=("order_id", "product_id"),\n )\nForeignObject is much like ForeignKey, except that it doesn’t create\nany columns (e.g. item_id), foreign key constraints or indexes in the\ndatabase, and the on_delete argument is ignored.
Many database functions only accept a single expression.
\nMAX("order_id") -- OK\nMAX("product_id", "order_id") -- ERROR\nIn these cases, providing a composite primary key reference raises a\nValueError, since it is composed of multiple column expressions. An\nexception is made for Count.
Max("order_id") # OK\nMax("pk") # ValueError\nCount("pk") # OK\nAs a composite primary key is a virtual field, a field which doesn’t represent\na single database column, this field is excluded from ModelForms.
\nFor example, take the following form:
\nclass OrderLineItemForm(forms.ModelForm):\n class Meta:\n model = OrderLineItem\n fields = "__all__"\nThis form does not have a form field pk for the composite primary key:
>>> OrderLineItemForm()\n<OrderLineItemForm bound=False, valid=Unknown, fields=(product;order;quantity)>\nSetting the primary composite field pk as a form field raises an unknown\nfield FieldError.
Since pk is only a virtual field, including pk as a field name in the\nexclude argument of Model.clean_fields() has no effect. To exclude\nthe composite primary key fields from\nmodel validation, specify each field individually.\nModel.validate_unique() can still be called with exclude={"pk"} to\nskip uniqueness checks.
Prior to the introduction of composite primary keys, the single field composing\nthe primary key of a model could be retrieved by introspecting the\nprimary key attribute of its\nfields:
>>> pk_field = None\n>>> for field in Product._meta.get_fields():\n... if field.primary_key:\n... pk_field = field\n... break\n...\n>>> pk_field\n<django.db.models.fields.AutoField: id>\nNow that a primary key can be composed of multiple fields the\nprimary key attribute can no\nlonger be relied upon to identify members of the primary key as it will be set\nto False to maintain the invariant that at most one field per model will\nhave this attribute set to True:
>>> pk_fields = []\n>>> for field in OrderLineItem._meta.get_fields():\n... if field.primary_key:\n... pk_fields.append(field)\n...\n>>> pk_fields\n[]\nIn order to build application code that properly handles composite primary\nkeys the _meta.pk_fields\nattribute should be used instead:
>>> Product._meta.pk_fields\n[<django.db.models.fields.AutoField: id>]\n>>> OrderLineItem._meta.pk_fields\n[\n <django.db.models.fields.ForeignKey: product>,\n <django.db.models.fields.ForeignKey: order>\n]\n