[1.8.x] Fixed #23617 -- Added get_pk_value_on_save()

The method is mainly intended for use with UUIDField. For UUIDField we
want to call the field's default even when primary key value is
explicitly set to None to match the behavior of AutoField.

Thanks to Marc Tamlyn and Tim Graham for review.

Backport of 8adc59038c from master

django/db/models/base.py

@@ -772,6 +772,9 @@ class Model(six.with_metaclass(ModelBase)):
                        if f.name in update_fields or f.attname in update_fields]
 
         pk_val = self._get_pk_val(meta)
+        if pk_val is None:
+            pk_val = meta.pk.get_pk_value_on_save(self)
+            setattr(self, meta.pk.attname, pk_val)
         pk_set = pk_val is not None
         if not pk_set and (force_update or update_fields):
             raise ValueError("Cannot force an update in save() with no primary key.")

django/db/models/fields/__init__.py

@@ -506,6 +506,17 @@ class Field(RegisterLookupMixin):
         return _load_field, (self.model._meta.app_label, self.model._meta.object_name,
                              self.name)
 
+    def get_pk_value_on_save(self, instance):
+        """
+        Hook to generate new PK values on save. This method is called when
+        saving instances with no primary key value set. If this method returns
+        something else than None, then the returned value is used when saving
+        the new instance.
+        """
+        if self.default:
+            return self.get_default()
+        return None
+
     def to_python(self, value):
         """
         Converts the input value into the expected Python data type, raising

django/db/models/query.py

@@ -345,6 +345,11 @@ class QuerySet(object):
         obj.save(force_insert=True, using=self.db)
         return obj
 
+    def _populate_pk_values(self, objs):
+        for obj in objs:
+            if obj.pk is None:
+                obj.pk = obj._meta.pk.get_pk_value_on_save(obj)
+
     def bulk_create(self, objs, batch_size=None):
         """
         Inserts each of the instances into the database. This does *not* call
@@ -369,6 +374,8 @@ class QuerySet(object):
         self._for_write = True
         connection = connections[self.db]
         fields = self.model._meta.local_concrete_fields
+        objs = list(objs)
+        self._populate_pk_values(objs)
         with transaction.atomic(using=self.db, savepoint=False):
             if (connection.features.can_combine_inserts_with_and_without_auto_increment_pk
                     and self.model._meta.has_auto_field):