Hi All,
I just discovered some interesting behaviour in Django 1.8.4 and was
wondering if this was an unexpected side effect or specifically designed
behaviour.
class UserProfile(models.Model):
# Notice that I do not say that blank=True
my_other_model = models.ForeignKey('my_app.OtherModel', null=True)
I would have expected that since it doesn't make sense for a ForeignKey to
have a 'blank' value, by saying that null=True it would implicitly mean
that blank=True.
However to my surprise this is not the case.
This came to my attention when using ModelForms. I found out that it is
possible to assign the foreign key in the Form to a non null value, but it
was not possible to reassign it back to the 'blank/null' value.
The reason is that in django/forms/models.py there is a function called
'_get_validation_exclusions'. This function does a check on line 376
if not f.blank and not form_field.required and field_value in
form_field.empty_values
which specifically looks at the 'blank' attribute of the model field. If
this is False, then you will go into the if block and your field will be
added to the exclusion list and therefore never saved.
To me, this seems like an unnecessary trap for users of the Django
Framework. However there may be a design decision here and I'd love to be
educated on why there would be a situation where it makes sense for
null=True and blank=False.
Regards
Ben
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