Fixed #29158 -- Fixed len(choices) crash if ModelChoiceField's queryset is a manager.

Removing all() in __iter__() prevents a duplicate query when choices are
cast to a list and there's a prefetch_related().

1 files changed, 2 insertions, 2 deletions

diff --git a/django/forms/models.py b/django/forms/models.py
index c546b3972a..8134cf3de3 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -1130,7 +1130,7 @@ class ModelChoiceIterator:
     def __iter__(self):
         if self.field.empty_label is not None:
             yield ("", self.field.empty_label)
-        queryset = self.queryset.all()
+        queryset = self.queryset
         # Can't use iterator() when queryset uses prefetch_related()
         if not queryset._prefetch_related_lookups:
             queryset = queryset.iterator()
@@ -1194,7 +1194,7 @@ class ModelChoiceField(ChoiceField):
         return self._queryset
 
     def _set_queryset(self, queryset):
-        self._queryset = queryset
+        self._queryset = None if queryset is None else queryset.all()
         self.widget.choices = self.choices
 
     queryset = property(_get_queryset, _set_queryset)