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Fixed #27039 -- Fixed fallback to model field default in model forms. #7068

merged 1 commit into from Aug 24, 2016
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Just for now

@@ -52,6 +52,11 @@ def construct_instance(form, instance, fields=None, exclude=None):
if exclude and in exclude:
# Leave defaults for fields that aren't in POST data, except for

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RobertAKARobin May 31, 2019


@timgraham This caused me a bit of a debugging headache. I expected that if I did something like this:

def clean(self):
    self.cleaned_data['some_field'] = 'some_value'

...that would be sufficient to set what was ultimately saved to the database. But I found that some_field was always being saved with its default value. It took me a while to figure out why, from looking at the Django source. Now I'm doing this, with success:

def __init__(self, *args, **kwargs):
    super(MyForm, self).__init__(*args, **kwargs) =['some_field'] = 'some_value'

I'm just curious what the reasoning is behind the rule you implemented here?

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timgraham May 31, 2019

Author Member

Your expectation seems reasonable. If there's a way to fix that while keeping backwards compatibility, feel free to offer a patch.

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RobertAKARobin May 31, 2019


Do Django's automated tests ensure backward compatibility? If not I probably don't have enough familiarity with Django to do so. Glad I'm not taking crazy pills, though. :)

# checkbox inputs because they don't appear in POST data if not checked.
if (f.has_default() and not in and
not getattr(form[].field.widget, 'dont_use_model_field_default_for_empty_data', False)):
# 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):
@@ -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
@@ -79,3 +79,6 @@ Bugfixes

* Reallowed subclassing ``UserCreationForm`` without ``USERNAME_FIELD`` in
``Meta.fields`` (:ticket:`27111`).

* Fixed a regression in model forms where model fields with a ``default`` that
didn't appear in POST data no longer used the ``default`` (:ticket:`27039`).
@@ -332,6 +332,21 @@ Note that if the form :ref:`hasn't been validated
``form.errors``. A ``ValueError`` will be raised if the data in the form
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` (or any custom widget with
``dont_use_model_field_default_for_empty_data=True``) 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 ``BooleanField``.

.. versionchanged:: 1.10.1

Older versions don't have the exception for
:class:`~django.forms.CheckboxInput` which means that unchecked checkboxes
receive a value of ``True`` if that's the model field default.

This ``save()`` method accepts an optional ``commit`` keyword argument, which
accepts either ``True`` or ``False``. If you call ``save()`` with
``commit=False``, then it will return an object that hasn't yet been saved to
@@ -122,6 +122,7 @@ class PublicationDefaults(models.Model):
date_published = models.DateField(
mode = models.CharField(max_length=2, choices=MODE_CHOICES, default=default_mode)
category = models.IntegerField(choices=CATEGORY_CHOICES, default=default_category)
active = models.BooleanField(default=True)

class Author(models.Model):
@@ -565,6 +565,42 @@ class Meta:
['slug', 'name'])

def test_default_populated_on_optional_field(self):
class PubForm(forms.ModelForm):
mode = forms.CharField(max_length=255, required=False)

class Meta:
model = PublicationDefaults
fields = ('mode',)

# Empty data uses the model field default.
mf1 = PubForm({})
self.assertEqual(mf1.errors, {})
m1 =
self.assertEqual(m1.mode, 'di')
self.assertEqual(m1._meta.get_field('mode').get_default(), 'di')

# Blank data doesn't use the model field default.
mf2 = PubForm({'mode': ''})
self.assertEqual(mf2.errors, {})
m2 =
self.assertEqual(m2.mode, '')

def test_default_not_populated_on_optional_checkbox_input(self):
class PubForm(forms.ModelForm):
class Meta:
model = PublicationDefaults
fields = ('active',)

# Empty data doesn't use the model default because CheckboxInput
# doesn't have a value in HTML form submission.
mf1 = PubForm({})
self.assertEqual(mf1.errors, {})
m1 =
self.assertIs(, False)
self.assertIsInstance(mf1.fields['active'].widget, forms.CheckboxInput)
self.assertIs(m1._meta.get_field('active').get_default(), True)

class FieldOverridesByFormMetaForm(forms.ModelForm):
class Meta:
@@ -774,7 +810,10 @@ def test_abstract_inherited_unique(self):
title = 'Boss'
isbn = '12345'
DerivedBook.objects.create(title=title, author=self.writer, isbn=isbn)
form = DerivedBookForm({'title': 'Other', 'author':, 'isbn': isbn})
form = DerivedBookForm({
'title': 'Other', 'author':, 'isbn': isbn,
'suffix1': '1', 'suffix2': '2',
self.assertEqual(len(form.errors), 1)
self.assertEqual(form.errors['isbn'], ['Derived book with this Isbn already exists.'])
@@ -2491,7 +2530,7 @@ def test_callable_field_default(self):
class PublicationDefaultsForm(forms.ModelForm):
class Meta:
model = PublicationDefaults
fields = '__all__'
fields = ('title', 'date_published', 'mode', 'category')

self.maxDiff = 2000
form = PublicationDefaultsForm()
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