[1.10.x] Fixed #27993 -- Fixed model form default fallback for SelectMultiple.

Backport of 7d1e23775344cc3dead03bd4af45f4fdf134b819 from master

3 files changed, 16 insertions, 10 deletions

diff --git a/docs/ref/forms/widgets.txt b/docs/ref/forms/widgets.txt
index b977513bad..71bebf26ef 100644
--- a/docs/ref/forms/widgets.txt
+++ b/docs/ref/forms/widgets.txt
@@ -281,11 +281,12 @@ foundation for custom widgets.
         The method's result affects whether or not a field in a model form
         :ref:`falls back to its default <topics-modelform-save>`.
 
-        Special cases are :class:`~django.forms.CheckboxInput` and
-        :class:`~django.forms.CheckboxSelectMultiple`, which always return
-        ``False`` because an unchecked checkbox doesn't appear in the data of
-        an HTML form submission, so it's unknown whether or not the user
-        actually submitted a value.
+        Special cases are :class:`~django.forms.CheckboxInput`,
+        :class:`~django.forms.CheckboxSelectMultiple`, and
+        :class:`~django.forms.SelectMultiple`, which always return
+        ``False`` because an unchecked checkbox and unselected
+        ``<select multiple>`` don't appear in the data of an HTML form
+        submission, so it's unknown whether or not the user submitted a value.
 
     .. method:: use_required_attribute(initial)
 
diff --git a/docs/releases/1.10.7.txt b/docs/releases/1.10.7.txt
index de9a511f07..9d4f79f5bf 100644
--- a/docs/releases/1.10.7.txt
+++ b/docs/releases/1.10.7.txt
@@ -11,3 +11,6 @@ Bugfixes
 
 * Made admin's ``RelatedFieldWidgetWrapper`` use the wrapped widget's
   ``value_omitted_from_data()`` method (:ticket:`27905`).
+
+* Fixed model form ``default`` fallback for ``SelectMultiple``
+  (:ticket:`27993`).
diff --git a/docs/topics/forms/modelforms.txt b/docs/topics/forms/modelforms.txt
index 768dc3938c..8f9828641a 100644
--- a/docs/topics/forms/modelforms.txt
+++ b/docs/topics/forms/modelforms.txt
@@ -335,12 +335,14 @@ doesn't validate -- i.e., if ``form.errors`` evaluates to ``True``.
 If an optional field doesn't appear in the form's data, the resulting model
 instance uses the model field :attr:`~django.db.models.Field.default`, if
 there is one, for that field. This behavior doesn't apply to fields that use
-:class:`~django.forms.CheckboxInput` and
-:class:`~django.forms.CheckboxSelectMultiple` (or any custom widget whose
+:class:`~django.forms.CheckboxInput`,
+:class:`~django.forms.CheckboxSelectMultiple`, or
+:class:`~django.forms.SelectMultiple` (or any custom widget whose
 :meth:`~django.forms.Widget.value_omitted_from_data` method always returns
-``False``) since an unchecked checkbox doesn't appear in the data of an HTML
-form submission. Use a custom form field or widget if you're designing an API
-and want the default fallback for a :class:`~django.db.models.BooleanField`.
+``False``) since an unchecked checkbox and unselected ``<select multiple>``
+don't appear in the data of an HTML form submission. Use a custom form field or
+widget if you're designing an API and want the default fallback behavior for a
+field that uses one of these widgets.
 
 .. versionchanged:: 1.10.1