Fixed #27039 -- Fixed empty data fallback to model field default in model forms.

django/forms/models.py

@@ -52,6 +52,11 @@ def construct_instance(form, instance, fields=None, exclude=None):
             continue
         if exclude and f.name in exclude:
             continue
+        # Leave defaults for fields that aren't in POST data, except for
+        # checkbox inputs because they don't appear in POST data if not checked.
+        if (f.has_default() and f.name not in form.data and
+                not getattr(form[f.name].field.widget, 'dont_use_model_field_default_for_empty_data', False)):
+            continue
         # Defer saving file-type fields until after the other fields, so a
         # callable upload_to can use the values from other fields.
         if isinstance(f, models.FileField):

django/forms/widgets.py

@@ -481,6 +481,10 @@ def boolean_check(v):
 
 
 class CheckboxInput(Widget):
+    # Don't use model field defaults for fields that aren't in POST data,
+    # because checkboxes don't appear in POST data if not checked.
+    dont_use_model_field_default_for_empty_data = True
+
     def __init__(self, attrs=None, check_test=None):
         super(CheckboxInput, self).__init__(attrs)
         # check_test is a callable that takes a value and returns True