[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

6 files changed, 34 insertions, 61 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
diff --git a/docs/releases/1.8.4.txt b/docs/releases/1.8.4.txt
index 77ca647ff0..b9d7c12c53 100644
--- a/docs/releases/1.8.4.txt
+++ b/docs/releases/1.8.4.txt
@@ -27,3 +27,7 @@ Bugfixes
 
 * Fixed the recording of squashed migrations when running the ``migrate``
   command (:ticket:`25231`).
+
+* Moved the :ref:`unsaved model instance assignment data loss check
+  <unsaved-model-instance-check-18>` to ``Model.save()`` to allow easier usage
+  of in-memory models (:ticket:`25160`).
diff --git a/docs/releases/1.8.txt b/docs/releases/1.8.txt
index 1901d7fba4..9f91d46340 100644
--- a/docs/releases/1.8.txt
+++ b/docs/releases/1.8.txt
@@ -684,9 +684,23 @@ 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.
 
+.. _unsaved-model-instance-check-18:
+
 Assigning unsaved objects to relations raises an error
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. note::
+
+    To more easily allow in-memory usage of models, this change was reverted in
+    Django 1.8.4 and replaced with a check during ``model.save()``. For example::
+
+        >>> book = Book.objects.create(name="Django")
+        >>> book.author = Author(name="John")
+        >>> book.save()
+        Traceback (most recent call last):
+        ...
+        ValueError: save() prohibited to prevent data loss due to unsaved related object 'author'.
+
 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`.
@@ -713,8 +727,8 @@ Now, an error will be raised to prevent data loss::
 If you require allowing the assignment of unsaved instances (the old behavior)
 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 using the
-:attr:`~django.db.models.ForeignKey.allow_unsaved_instance_assignment`
-attribute.
+``ForeignKey.allow_unsaved_instance_assignment`` attribute. (This attribute was
+removed in 1.8.4 as it's no longer relevant.)
 
 Management commands that only accept positional arguments
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/docs/topics/db/examples/many_to_one.txt b/docs/topics/db/examples/many_to_one.txt
index e43809fd1c..63b700c5a5 100644
--- a/docs/topics/db/examples/many_to_one.txt
+++ b/docs/topics/db/examples/many_to_one.txt
@@ -55,19 +55,17 @@ 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)
+    >>> Article.objects.create(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.'
+    ValueError: save() prohibited to prevent data loss due to unsaved related object 'reporter'.
 
-If you want to disable the unsaved instance check, you can use the
-:attr:`~django.db.models.ForeignKey.allow_unsaved_instance_assignment`
-attribute.
+.. versionchanged:: 1.8.4
 
-.. versionchanged:: 1.8
-
-    Previously, assigning unsaved objects did not raise an error and could
-    result in silent data loss.
+    Previously, saving an object with unsaved related objects did not raise an
+    error and could result in silent data loss. In 1.8-1.8.3, unsaved model
+    instances couldn't be assigned to related fields, but this restriction was
+    removed to allow easier usage of in-memory models.
 
 Article objects have access to their related Reporter objects::
 
diff --git a/docs/topics/db/examples/one_to_one.txt b/docs/topics/db/examples/one_to_one.txt
index f2a3f54508..1cb7073437 100644
--- a/docs/topics/db/examples/one_to_one.txt
+++ b/docs/topics/db/examples/one_to_one.txt
@@ -92,23 +92,17 @@ 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)
+    >>> Restaurant.objects.create(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.'
-
-If you want to disable the unsaved instance check, you can use the
-:attr:`~django.db.models.ForeignKey.allow_unsaved_instance_assignment`
-attribute.
+    ValueError: save() prohibited to prevent data loss due to unsaved related object 'place'.
 
-.. versionchanged:: 1.8
+.. versionchanged:: 1.8.4
 
-    Previously, assigning unsaved objects did not raise an error and could
-    result in silent data loss.
+    Previously, saving an object with unsaved related objects did not raise an
+    error and could result in silent data loss. In 1.8-1.8.3, unsaved model
+    instances couldn't be assigned to related fields, but this restriction was
+    removed to allow easier usage of in-memory models.
 
 Restaurant.objects.all() just returns the Restaurants, not the Places.  Note
 that there are two restaurants - Ace Hardware the Restaurant was created in the