[1.7.x] Fixed #23609 -- Fixed IntegrityError that prevented altering a NULL column into a NOT NULL one due to existing rows

Thanks to Simon Charette, Loic Bistuer and Tim Graham for the review.

Backport of f633ba778d from master

2 files changed, 18 insertions, 2 deletions

diff --git a/docs/ref/migration-operations.txt b/docs/ref/migration-operations.txt
index e377a251f4..e5c21ea072 100644
--- a/docs/ref/migration-operations.txt
+++ b/docs/ref/migration-operations.txt
@@ -137,7 +137,7 @@ or if it is temporary and just for this migration (``False``) - usually
 because the migration is adding a non-nullable field to a table and needs
 a default value to put into existing rows. It does not effect the behavior
 of setting defaults in the database directly - Django never sets database
-defaults, and always applies them in the Django ORM code.
+defaults and always applies them in the Django ORM code.
 
 RemoveField
 -----------
@@ -153,16 +153,28 @@ from any data loss, which of course is irreversible).
 AlterField
 ----------
 
-.. class:: AlterField(model_name, name, field)
+.. class:: AlterField(model_name, name, field, preserve_default=True)
 
 Alters a field's definition, including changes to its type,
 :attr:`~django.db.models.Field.null`, :attr:`~django.db.models.Field.unique`,
 :attr:`~django.db.models.Field.db_column` and other field attributes.
 
+The ``preserve_default`` argument indicates whether the field's default
+value is permanent and should be baked into the project state (``True``),
+or if it is temporary and just for this migration (``False``) - usually
+because the migration is altering a nullable field to a non-nullable one and
+needs a default value to put into existing rows. It does not effect the
+behavior of setting defaults in the database directly - Django never sets
+database defaults and always applies them in the Django ORM code.
+
 Note that not all changes are possible on all databases - for example, you
 cannot change a text-type field like ``models.TextField()`` into a number-type
 field like ``models.IntegerField()`` on most databases.
 
+.. versionchanged:: 1.7.1
+
+    The ``preserve_default`` argument was added.
+
 RenameField
 -----------
 
diff --git a/docs/releases/1.7.1.txt b/docs/releases/1.7.1.txt
index 68370dd44a..70535b8161 100644
--- a/docs/releases/1.7.1.txt
+++ b/docs/releases/1.7.1.txt
@@ -108,3 +108,7 @@ Bugfixes
   type (byte string) on Python 3 (:ticket:`23333`).
 
 * :djadmin:`makemigrations` can now serialize timezone-aware values (:ticket:`23365`).
+
+* Added a prompt to the migrations questioner when removing the null constraint
+  from a field to prevent an IntegrityError on existing NULL rows
+  (:ticket:`23609`).