[1.8.x] Fixed #25160 -- Moved unsaved model instance data loss check to Model.save()

This mostly reverts 5643a3b51be338196d0b292d5626ad43648448d3 and
81e1a35c364e5353d2bf99368ad30a4184fbb653.

Thanks Carl Meyer for review.

Backport of 5980b05c1fad69eef907e0076aa2dc837edab529 from master

2 files changed, 0 insertions, 37 deletions

diff --git a/docs/ref/contrib/contenttypes.txt b/docs/ref/contrib/contenttypes.txt
index 7dce890b15..510c6b2dc4 100644
--- a/docs/ref/contrib/contenttypes.txt
+++ b/docs/ref/contrib/contenttypes.txt
@@ -299,13 +299,6 @@ model:
        is ``True``. This mirrors the ``for_concrete_model`` argument to
        :meth:`~django.contrib.contenttypes.models.ContentTypeManager.get_for_model`.
 
-    .. attribute:: GenericForeignKey.allow_unsaved_instance_assignment
-
-        .. versionadded:: 1.8
-
-        Works analogously to :attr:`ForeignKey.allow_unsaved_instance_assignment
-        <django.db.models.ForeignKey.allow_unsaved_instance_assignment>`.
-
     .. deprecated:: 1.7
 
         This class used to be defined in ``django.contrib.contenttypes.generic``.
diff --git a/docs/ref/models/fields.txt b/docs/ref/models/fields.txt
index 8f66fc5a53..7e2bde56c3 100644
--- a/docs/ref/models/fields.txt
+++ b/docs/ref/models/fields.txt
@@ -1349,30 +1349,6 @@ The possible values for :attr:`~ForeignKey.on_delete` are found in
 
     If in doubt, leave it to its default of ``True``.
 
-.. attribute:: ForeignKey.allow_unsaved_instance_assignment
-
-    .. versionadded:: 1.8
-
-        This flag was added for backwards compatibility as older versions of
-        Django always allowed assigning unsaved model instances.
-
-    Django prevents unsaved model instances from being assigned to a
-    ``ForeignKey`` field to prevent accidental data loss (unsaved foreign keys
-    are silently ignored when saving a model instance).
-
-    If you require allowing the assignment of unsaved instances and aren't
-    concerned about the data loss possibility (e.g. you never save the objects
-    to the database), you can disable this check by creating a subclass of the
-    field class and setting its ``allow_unsaved_instance_assignment`` attribute
-    to ``True``. For example::
-
-        class UnsavedForeignKey(models.ForeignKey):
-            # A ForeignKey which can point to an unsaved object
-            allow_unsaved_instance_assignment = True
-
-        class Book(models.Model):
-            author = UnsavedForeignKey(Author)
-
 .. _ref-manytomany:
 
 ``ManyToManyField``
@@ -1575,12 +1551,6 @@ that control how the relationship functions.
 
     If in doubt, leave it to its default of ``True``.
 
-.. attribute:: ManyToManyField.allow_unsaved_instance_assignment
-
-    .. versionadded:: 1.8
-
-    Works analogously to :attr:`ForeignKey.allow_unsaved_instance_assignment`.
-
 :class:`ManyToManyField` does not support :attr:`~Field.validators`.
 
 :attr:`~Field.null` has no effect since there is no way to require a