Comparison of three ways of assigning excluded fields to a model when creating via a form

comparison.py

# Question - how best to handle model fields that are excluded from a form's
# fields.

# ------------------------------------------------------------
# Option 1: assign fields to instance before passing into form
# ------------------------------------------------------------

# views.py
instance = Vote(review=review, user=request.user)
form = VoteForm(data=request.POST, instance=instance)

# --------------------------------------------------------
# Option 2: pass excluded field values to form constructor
# --------------------------------------------------------

# views.py
form = VoteForm(data=request.POST, review=review, user=request.user,
                instance=instance)

# forms.py
class VoteForm(forms.ModelForm):

    def __init__(self, review, user, *args, **kwargs):
        super(VoteForm, self).__init__(*args, **kwargs)
        self.instance.review = review
        self.instance.user = user

# -----------------------------------------------------------------------------
# Option 3: pass excluded field values to form constructor but defer assignment
# -----------------------------------------------------------------------------

# views.py
form = VoteForm(data=request.POST, review=review, user=request.user,
                instance=instance)

# forms.py
class VoteForm(forms.ModelForm):

    def __init__(self, review, user, *args, **kwargs):
        self.review = review
        self.user = user
        super(VoteForm, self).__init__(*args, **kwargs)

    def save(self, commit=True):
        vote = super(VoteForm, self).save(commit=False)
        vote.review = self.review
        vote.user = self.user
        if commit:
            vote.save()
        return vote

# (This method is bad as the model's clean method doesn't have access to the
# review and user instances).

# Option 1 is best right? (Confession - I've been using option 2 for ages)

Also I vote for comment on #1 by @AndrewIngram:

Anyway, if you have to choose between Option 1 and 2, I'd go with 2. With option 1 you can't just look at the form's init method and see that we require a review and a user just to make the form work.

Very well noted. But there are also many cases where form actually doesn't need those excluded fields to work in this case #1 should be fine.

For the record, I think option 2 is better for Oscar as the API is simpler (as @AndrewIngram pointed out). It's more cryptic when you have to assign things to the instance argument.

However, option 1 is slightly easier to unit test as you don't have to create instances of a review and user to pass to the constructor.

Assuming you're using factories, I'm not sure it's much of a problem to generate a review and a user.

Alternatively, just pass in None if your test doesn't hit the part of the code where review and user are actually required to be anything in particular. And if it does hit those parts, you're gonna need to construct proper instances anyway.