I will best explain what I mean with an example.
Lets say we have a simple model called Book.
class Book(models.Model):
user = models.ForeignKey(User)
name = models.CharField(max_length=255)
class Meta:
unique_together = ('user', 'name')
so `Books` are linked to users, and we have added an unique constrain
because we don't want a user to have the same book twice.
Now lets say we want to allow users to create/update books, when we do this
we would only need their book.name, we already knew who they are in the
system.
So a simple ModelForm would look like this:
class BookForm(forms.ModelForm):
class Meta:
model = Book
fields = ('name',)
Now since i've omitted the `user` field and I can't have a book w/o a user,
its my responsibility to populate it. Thus when I create new form i will
always a pass an instance with a user.
But before we do this, lets manually create 1 book for this user.
user = User.objects.get(pk=1)
book1 = Book.objects.create(user=user, name='Harry Potter')
#now lets build our form
book2 = Book(user=user)
form = BookForm(data={'name': 'Harry Potter'}, instance=book2)
if form.is_valid(): #yes the form will be valid
book2 = form.save() #IntegrityError will be raised with UNIQUE
constraint failed
Now this happens because `ModelForm.validate_unique
<https://github.com/django/django/blob/1.8.18/django/forms/models.py#L434>`
collects a set of fields to pass as an exclude fields into
`instance.validate_unique`.
Please, can someone explain me why is this done in such a manner, and what
will be the correct way approach this problem.
>From my point of view when I define a ModelForm I imagine that I have to
only explain what the user needs to see when the form gets rendered.
But when the instance is being validated, I would always want to validate
this instance as a whole (with the fields that I've omitted).
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