Fixed #31295 -- Avoided Select widget triggering additional query when using ModelChoiceIterator.fix-31295

1 files changed, 19 insertions, 7 deletions

diff --git a/django/forms/models.py b/django/forms/models.py
index 9686baa6f2..c04a806caa 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -3,6 +3,7 @@ Helper functions for creating Form classes from Django models
 and database field objects.
 """
 
+from collections import deque
 from itertools import chain
 
 from django.core.exceptions import (
@@ -1438,17 +1439,20 @@ class ModelChoiceIterator(BaseChoiceIterator):
     def __init__(self, field):
         self.field = field
         self.queryset = field.queryset
+        self._deque = deque()
+        self._generator = self.generator()
 
-    def __iter__(self):
+    def generator(self):
         if self.field.empty_label is not None:
             yield ("", self.field.empty_label)
-        queryset = self.queryset
-        # Can't use iterator() when queryset uses prefetch_related()
-        if not queryset._prefetch_related_lookups:
-            queryset = queryset.iterator()
-        for obj in queryset:
+        for obj in self.queryset.iterator(chunk_size=100):
             yield self.choice(obj)
 
+    def __iter__(self):
+        yield from chain(self._deque, self._generator)
+        self._deque = deque()
+        self._generator = self.generator()
+
     def __len__(self):
         # count() adds a query but uses less memory since the QuerySet results
         # won't be cached. In most cases, the choices will only be iterated on,
@@ -1456,7 +1460,9 @@ class ModelChoiceIterator(BaseChoiceIterator):
         return self.queryset.count() + (1 if self.field.empty_label is not None else 0)
 
     def __bool__(self):
-        return self.field.empty_label is not None or self.queryset.exists()
+        return (
+            self.field.empty_label is not None or self._deque or self.queryset.exists()
+        )
 
     def choice(self, obj):
         return (
@@ -1464,6 +1470,12 @@ class ModelChoiceIterator(BaseChoiceIterator):
             self.field.label_from_instance(obj),
         )
 
+    def peek(self):
+        value = next(self._generator, None)
+        if value is not None:
+            self._deque.append(value)
+        return value
+
 
 class ModelChoiceField(ChoiceField):
     """A ChoiceField whose choices are a model QuerySet."""