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/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