Added a db_constraint option to ForeignKeys.

This controls whether or not a database level cosntraint is created. This is useful in a few specialized circumstances, but in general should not be used!

django/db/backends/creation.py

@@ -77,7 +77,7 @@ class BaseDatabaseCreation(object):
                     tablespace, inline=True)
                 if tablespace_sql:
                     field_output.append(tablespace_sql)
-            if f.rel:
+            if f.rel and f.db_constraint:
                 ref_output, pending = self.sql_for_inline_foreign_key_references(
                     model, f, known_models, style)
                 if pending:

django/db/models/fields/related.py

@@ -981,9 +981,10 @@ class ForeignKey(RelatedField, Field):
     }
     description = _("Foreign Key (type determined by related field)")
 
-    def __init__(self, to, to_field=None, rel_class=ManyToOneRel, **kwargs):
+    def __init__(self, to, to_field=None, rel_class=ManyToOneRel,
+                 db_constraint=True, **kwargs):
         try:
-            to_name = to._meta.model_name
+            to._meta.model_name
         except AttributeError:  # to._meta doesn't exist, so it must be RECURSIVE_RELATIONSHIP_CONSTANT
             assert isinstance(to, six.string_types), "%s(%r) is invalid. First parameter to ForeignKey must be either a model, a model name, or the string %r" % (self.__class__.__name__, to, RECURSIVE_RELATIONSHIP_CONSTANT)
         else:
@@ -997,13 +998,14 @@ class ForeignKey(RelatedField, Field):
         if 'db_index' not in kwargs:
             kwargs['db_index'] = True
 
+        self.db_constraint = db_constraint
         kwargs['rel'] = rel_class(to, to_field,
             related_name=kwargs.pop('related_name', None),
             limit_choices_to=kwargs.pop('limit_choices_to', None),
             parent_link=kwargs.pop('parent_link', False),
             on_delete=kwargs.pop('on_delete', CASCADE),
         )
-        Field.__init__(self, **kwargs)
+        super(ForeignKey, self).__init__(**kwargs)
 
     def get_path_info(self):
         """

docs/ref/models/fields.txt

@@ -1051,6 +1051,19 @@ define the details of how the relation works.
     The field on the related object that the relation is to. By default, Django
     uses the primary key of the related object.
 
+.. attribute:: ForeignKey.db_constraint
+
+    Controls whether or not a constraint should be created in the database for
+    this foreign key. The default is ``True``, and that's almost certainly what
+    you want; setting this to ``False`` can be very bad for data integrity.
+    That said, here are some scenarios where you might want to do this:
+
+    * You have legacy data that is not valid.
+    * You're sharding your database.
+
+    If you use this, accessing a related object that doesn't exist will raise
+    its ``DoesNotExist`` exception.
+
 .. attribute:: ForeignKey.on_delete
 
     When an object referenced by a :class:`ForeignKey` is deleted, Django by

docs/releases/1.6.txt

@@ -85,6 +85,9 @@ Minor features
   :class:`~django.http.HttpResponsePermanentRedirect` now provide an ``url``
   attribute (equivalent to the URL the response will redirect to).
 
+* Added the :attr:`django.db.models.ForeignKey.db_constraint`
+  option.
+
 Backwards incompatible changes in 1.6
 =====================================
 

tests/regressiontests/backends/models.py

@@ -39,7 +39,7 @@ if connection.features.supports_long_model_names:
             verbose_name = 'model_with_long_table_name'
         primary_key_is_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz = models.AutoField(primary_key=True)
         charfield_is_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz = models.CharField(max_length=100)
-        m2m_also_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz = models.ManyToManyField(Person,blank=True)
+        m2m_also_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz = models.ManyToManyField(Person, blank=True)
 
 
 class Tag(models.Model):
@@ -86,3 +86,16 @@ class Item(models.Model):
 
     def __str__(self):
         return self.name
+
+
+@python_2_unicode_compatible
+class Object(models.Model):
+    pass
+
+
+@python_2_unicode_compatible
+class ObjectReference(models.Model):
+    obj = models.ForeignKey(Object, db_constraint=False)
+
+    def __str__(self):
+        return str(self.obj_id)

tests/regressiontests/backends/tests.py

@@ -7,13 +7,12 @@ import threading
 
 from django.conf import settings
 from django.core.management.color import no_style
-from django.core.exceptions import ImproperlyConfigured
 from django.db import (backend, connection, connections, DEFAULT_DB_ALIAS,
     IntegrityError, transaction)
 from django.db.backends.signals import connection_created
 from django.db.backends.postgresql_psycopg2 import version as pg_version
-from django.db.models import fields, Sum, Avg, Variance, StdDev
-from django.db.utils import ConnectionHandler, DatabaseError, load_backend
+from django.db.models import Sum, Avg, Variance, StdDev
+from django.db.utils import ConnectionHandler, DatabaseError
 from django.test import (TestCase, skipUnlessDBFeature, skipIfDBFeature,
     TransactionTestCase)
 from django.test.utils import override_settings, str_prefix
@@ -724,3 +723,22 @@ class MySQLPKZeroTests(TestCase):
     def test_zero_as_autoval(self):
         with self.assertRaises(ValueError):
             models.Square.objects.create(id=0, root=0, square=1)
+
+
+class DBConstraintTestCase(TransactionTestCase):
+    def test_can_reference_existant(self):
+        obj = models.Object.objects.create()
+        ref = models.ObjectReference.objects.create(obj=obj)
+        self.assertEqual(ref.obj, obj)
+
+        ref = models.ObjectReference.objects.get(obj=obj)
+        self.assertEqual(ref.obj, obj)
+
+    def test_can_reference_non_existant(self):
+        self.assertFalse(models.Object.objects.filter(id=12345).exists())
+        ref = models.ObjectReference.objects.create(obj_id=12345)
+        ref_new = models.ObjectReference.objects.get(obj_id=12345)
+        self.assertEqual(ref, ref_new)
+
+        with self.assertRaises(models.Object.DoesNotExist):
+            ref.obj