Fixed #10811 -- Made assigning unsaved objects to FK, O2O, and GFK raise ValueError.

This prevents silent data loss.

Thanks Aymeric Augustin for the initial patch and Loic Bistuer for the review.

docs/releases/1.8.txt

@@ -218,19 +218,47 @@ Backwards incompatible changes in 1.8
     deprecation timeline for a given feature, its removal may appear as a
     backwards incompatible change.
 
-* Some operations on related objects such as
-  :meth:`~django.db.models.fields.related.RelatedManager.add()` or
-  :ref:`direct assignment<direct-assignment>` ran multiple data modifying
-  queries without wrapping them in transactions. To reduce the risk of data
-  corruption, all data modifying methods that affect multiple related objects
-  (i.e. ``add()``, ``remove()``, ``clear()``, and
-  :ref:`direct assignment<direct-assignment>`) now perform their data modifying
-  queries from within a transaction, provided your database supports
-  transactions.
+Related object operations are run in a transaction
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-  This has one backwards incompatible side effect, signal handlers triggered
-  from these methods are now executed within the method's transaction and
-  any exception in a signal handler will prevent the whole operation.
+Some operations on related objects such as
+:meth:`~django.db.models.fields.related.RelatedManager.add()` or
+:ref:`direct assignment<direct-assignment>` ran multiple data modifying
+queries without wrapping them in transactions. To reduce the risk of data
+corruption, all data modifying methods that affect multiple related objects
+(i.e. ``add()``, ``remove()``, ``clear()``, and :ref:`direct assignment
+<direct-assignment>`) now perform their data modifying queries from within a
+transaction, provided your database supports transactions.
+
+This has one backwards incompatible side effect, signal handlers triggered from
+these methods are now executed within the method's transaction and any
+exception in a signal handler will prevent the whole operation.
+
+Unassigning unsaved objects to relations raises an error
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Assigning unsaved objects to a :class:`~django.db.models.ForeignKey`,
+:class:`~django.contrib.contenttypes.fields.GenericForeignKey`, and
+:class:`~django.db.models.OneToOneField` now raises a :exc:`ValueError`.
+
+Previously, the assignment of an unsaved object would be silently ignored.
+For example::
+
+    >>> book = Book.objects.create(name="Django")
+    >>> book.author = Author(name="John")
+    >>> book.author.save()
+    >>> book.save()
+
+    >>> Book.objects.get(name="Django")
+    >>> book.author
+    >>>
+
+Now, an error will be raised to prevent data loss::
+
+    >>> book.author = Author(name="john")
+    Traceback (most recent call last):
+    ...
+    ValueError: Cannot assign "<Author: John>": "Author" instance isn't saved in the database.
 
 Miscellaneous
 ~~~~~~~~~~~~~

docs/topics/db/examples/many_to_one.txt

@@ -52,6 +52,21 @@ Create an Article::
     >>> a.reporter
     <Reporter: John Smith>
 
+Note that you must save an object before it can be assigned to a foreign key
+relationship. For example, creating an ``Article`` with unsaved ``Reporter``
+raises ``ValueError``::
+
+    >>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')
+    >>> Article(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)
+    Traceback (most recent call last):
+    ...
+    ValueError: 'Cannot assign "<Reporter: John Smith>": "Reporter" instance isn't saved in the database.'
+
+.. versionchanged:: 1.8
+
+    Previously, assigning unsaved objects did not raise an error and could
+    result in silent data loss.
+
 Article objects have access to their related Reporter objects::
 
     >>> r = a.reporter

docs/topics/db/examples/one_to_one.txt

@@ -89,6 +89,25 @@ Set the place back again, using assignment in the reverse direction::
     >>> p1.restaurant
     <Restaurant: Demon Dogs the restaurant>
 
+Note that you must save an object before it can be assigned to a one-to-one
+relationship. For example, creating an ``Restaurant`` with unsaved ``Place``
+raises ``ValueError``::
+
+    >>> p3 = Place(name='Demon Dogs', address='944 W. Fullerton')
+    >>> Restaurant(place=p3, serves_hot_dogs=True, serves_pizza=False)
+    Traceback (most recent call last):
+    ...
+    ValueError: 'Cannot assign "<Place: Demon Dogs>": "Place" instance isn't saved in the database.'
+    >>> p.restaurant = Restaurant(place=p, serves_hot_dogs=True, serves_pizza=False)
+    Traceback (most recent call last):
+    ...
+    ValueError: 'Cannot assign "<Restaurant: Demon Dogs the restaurant>": "Restaurant" instance isn't saved in the database.'
+
+.. versionchanged:: 1.8
+
+    Previously, assigning unsaved objects did not raise an error and could
+    result in silent data loss.
+
 Restaurant.objects.all() just returns the Restaurants, not the Places.  Note
 that there are two restaurants - Ace Hardware the Restaurant was created in the
 call to r.place = p2::