[1.11.x] Fixed #28159 -- Fixed BaseInlineFormSet._construct_form() crash when using save_as_new.

Regression in 4a246a02bdcbc13b15480c014f51cb0682af7c1e.

Backport of 362fba87c9bb4f88542ba82ce4a732fed2634be7 from master

3 files changed, 16 insertions, 3 deletions

diff --git a/django/forms/models.py b/django/forms/models.py
index af324c2b27..ed9df60321 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -894,12 +894,17 @@ class BaseInlineFormSet(BaseModelFormSet):
     def _construct_form(self, i, **kwargs):
         form = super(BaseInlineFormSet, self)._construct_form(i, **kwargs)
         if self.save_as_new:
+            mutable = getattr(form.data, '_mutable', None)
+            # Allow modifying an immutable QueryDict.
+            if mutable is not None:
+                form.data._mutable = True
             # Remove the primary key from the form's data, we are only
             # creating new instances
             form.data[form.add_prefix(self._pk_field.name)] = None
-
             # Remove the foreign key from the form's data
             form.data[form.add_prefix(self.fk.name)] = None
+            if mutable is not None:
+                form.data._mutable = mutable
 
         # Set the fk value here so that the form can do its validation.
         fk_value = self.instance.pk
diff --git a/docs/releases/1.11.1.txt b/docs/releases/1.11.1.txt
index 9a9d96bf46..727ce94751 100644
--- a/docs/releases/1.11.1.txt
+++ b/docs/releases/1.11.1.txt
@@ -81,3 +81,6 @@ Bugfixes
 
 * Fixed a regression in choice ordering in form fields with grouped and
   non-grouped options (:ticket:`28157`).
+
+* Fixed crash in  ``BaseInlineFormSet._construct_form()`` when using
+  ``save_as_new`` (:ticket:`28159`).
diff --git a/tests/model_formsets/tests.py b/tests/model_formsets/tests.py
index 4387fbf52d..bc8cf1cac3 100644
--- a/tests/model_formsets/tests.py
+++ b/tests/model_formsets/tests.py
@@ -12,6 +12,7 @@ from django.forms.models import (
     BaseModelFormSet, _get_foreign_key, inlineformset_factory,
     modelformset_factory,
 )
+from django.http import QueryDict
 from django.test import TestCase, skipUnlessDBFeature
 from django.utils import six
 
@@ -702,7 +703,9 @@ class ModelFormsetTest(TestCase):
         AuthorBooksFormSet = inlineformset_factory(Author, Book, can_delete=False, extra=2, fields="__all__")
         Author.objects.create(name='Charles Baudelaire')
 
-        data = {
+        # An immutable QueryDict simulates request.POST.
+        data = QueryDict(mutable=True)
+        data.update({
             'book_set-TOTAL_FORMS': '3',  # the number of forms rendered
             'book_set-INITIAL_FORMS': '2',  # the number of forms with initial data
             'book_set-MAX_NUM_FORMS': '',  # the max number of forms
@@ -711,10 +714,12 @@ class ModelFormsetTest(TestCase):
             'book_set-1-id': '2',
             'book_set-1-title': 'Les Paradis Artificiels',
             'book_set-2-title': '',
-        }
+        })
+        data._mutable = False
 
         formset = AuthorBooksFormSet(data, instance=Author(), save_as_new=True)
         self.assertTrue(formset.is_valid())
+        self.assertIs(data._mutable, False)
 
         new_author = Author.objects.create(name='Charles Baudelaire')
         formset = AuthorBooksFormSet(data, instance=new_author, save_as_new=True)