[1.11.x] Fixed #27758 -- Reallowed AdvancedModelIterator pattern after template widget rendering.

Backport of 6d8979f4c2 from master

django/forms/widgets.py

@@ -569,8 +569,6 @@ class ChoiceWidget(Widget):
         for option_value, option_label in chain(self.choices):
             if option_value is None:
                 option_value = ''
-            else:
-                option_value = force_text(option_value)
 
             if isinstance(option_label, (list, tuple)):
                 index = groups[-1][2] + 1
@@ -586,7 +584,7 @@ class ChoiceWidget(Widget):
 
             for subvalue, sublabel in choices:
                 selected = (
-                    subvalue in value and
+                    force_text(subvalue) in value and
                     (has_selected is False or self.allow_multiple_selected)
                 )
                 if selected is True and has_selected is False:

tests/model_forms/tests.py

@@ -17,6 +17,7 @@ from django.forms.models import (
     ModelChoiceIterator, ModelFormMetaclass, construct_instance,
     fields_for_model, model_to_dict, modelform_factory,
 )
+from django.forms.widgets import CheckboxSelectMultiple
 from django.template import Context, Template
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 from django.utils import six
@@ -1708,6 +1709,44 @@ class ModelChoiceFieldTests(TestCase):
         field = CustomModelChoiceField(Category.objects.all())
         self.assertIsInstance(field.choices, CustomModelChoiceIterator)
 
+    def test_modelchoicefield_iterator_pass_model_to_widget(self):
+        class CustomModelChoiceValue:
+            def __init__(self, value, obj):
+                self.value = value
+                self.obj = obj
+
+            def __str__(self):
+                return str(self.value)
+
+        class CustomModelChoiceIterator(ModelChoiceIterator):
+            def choice(self, obj):
+                value, label = super(CustomModelChoiceIterator, self).choice(obj)
+                return CustomModelChoiceValue(value, obj), label
+
+        class CustomCheckboxSelectMultiple(CheckboxSelectMultiple):
+            def create_option(self, name, value, label, selected, index, subindex=None, attrs=None):
+                option = super(CustomCheckboxSelectMultiple, self).create_option(
+                    name, value, label, selected, index, subindex=None, attrs=None,
+                )
+                # Modify the HTML based on the object being rendered.
+                c = value.obj
+                option['attrs']['data-slug'] = c.slug
+                return option
+
+        class CustomModelMultipleChoiceField(forms.ModelMultipleChoiceField):
+            iterator = CustomModelChoiceIterator
+            widget = CustomCheckboxSelectMultiple
+
+        field = CustomModelMultipleChoiceField(Category.objects.all())
+        self.assertHTMLEqual(
+            field.widget.render('name', []),
+            '''<ul>
+<li><label><input type="checkbox" name="name" value="%d" data-slug="entertainment" />Entertainment</label></li>
+<li><label><input type="checkbox" name="name" value="%d" data-slug="its-test" />It&#39;s a test</label></li>
+<li><label><input type="checkbox" name="name" value="%d" data-slug="third-test" />Third</label></li>
+</ul>''' % (self.c1.pk, self.c2.pk, self.c3.pk),
+        )
+
     def test_modelchoicefield_num_queries(self):
         """
         Widgets that render multiple subwidgets shouldn't make more than one