[1.8.x] Fixed #24395 -- Ensured inline ModelsForms have an updated related instance.

Backport of 4c2f546b55 from master

django/forms/models.py

@@ -891,6 +891,10 @@ class BaseInlineFormSet(BaseModelFormSet):
         return cls.fk.rel.get_accessor_name(model=cls.model).replace('+', '')
 
     def save_new(self, form, commit=True):
+        # Ensure the latest copy of the related instance is present on each
+        # form (it may have been saved after the formset was originally
+        # instantiated).
+        setattr(form.instance, self.fk.name, self.instance)
         # Use commit=False so we can assign the parent key afterwards, then
         # save the object.
         obj = form.save(commit=False)

tests/model_formsets/tests.py

@@ -819,6 +819,38 @@ class ModelFormsetTest(TestCase):
         formset = AuthorBooksFormSet(data, instance=author, queryset=custom_qs)
         self.assertTrue(formset.is_valid())
 
+    def test_inline_formsets_with_custom_save_method_related_instance(self):
+        """
+        The ModelForm.save() method should be able to access the related object
+        if it exists in the database (#24395).
+        """
+        class PoemForm2(forms.ModelForm):
+            def save(self, commit=True):
+                poem = super(PoemForm2, self).save(commit=False)
+                poem.name = "%s by %s" % (poem.name, poem.poet.name)
+                if commit:
+                    poem.save()
+                return poem
+
+        PoemFormSet = inlineformset_factory(Poet, Poem, form=PoemForm2, fields="__all__")
+        data = {
+            'poem_set-TOTAL_FORMS': '1',
+            'poem_set-INITIAL_FORMS': '0',
+            'poem_set-MAX_NUM_FORMS': '',
+            'poem_set-0-name': 'Le Lac',
+        }
+        poet = Poet()
+        formset = PoemFormSet(data=data, instance=poet)
+        self.assertTrue(formset.is_valid())
+
+        # The Poet instance is saved after the formset instantiation. This
+        # happens in admin's changeform_view() when adding a new object and
+        # some inlines in the same request.
+        poet.name = 'Lamartine'
+        poet.save()
+        poem = formset.save()[0]
+        self.assertEqual(poem.name, 'Le Lac by Lamartine')
+
     def test_inline_formsets_with_wrong_fk_name(self):
         """ Regression for #23451 """
         message = "fk_name 'title' is not a ForeignKey to 'model_formsets.Author'."