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

Regression in 4a246a02bdcbc13b15480c014f51cb0682af7c1e.

Backport of 362fba87c9bb4f88542ba82ce4a732fed2634be7 from master

1 files changed, 6 insertions, 1 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