Set instance of each form in Django model formset

Question

When working with Django model forms, I often do something like this:

def my_view(request):
    new_entry = MyModel(name='a')
    form = MyModelForm(instance=new_entry)
    ...

I want to do something similar with a modelformset. Something like this would be ideal:

def my_view(request):
    MyFormSet = modelformset_factory(MyModel, form=MyModelForm)
    new_entries = [MyModel(name='a'), MyModel(name='b')]
    formset = MyFormSet(instances=new_entries) # off course this does not work
    ...

Since the items are not stored in the database yet, I can't set the instances using a queryset. If I want to use initial I have to declare the fields in the form, which is not ideal and seems a bit hacky.

Any suggestions how I can set the instances of each modelform in a modelformset?

Answer 1

Ok, I think I've found a solution.

class FormSetWithInstances(BaseFormSet):
    def __init__(self, *args, **kwargs):
        self.instances = kwargs.pop('instances')
        super(FormSetWithInstances, self).__init__(*args, **kwargs)

    def get_form_kwargs(self, index):
        form_kwargs = super(FormSetWithInstances, self).get_form_kwargs(index)
        if index < len(self.instances):
            form_kwargs['instance'] = self.instances[index]
        return form_kwargs

Be careful when using this modelformsets or inlinemodelformsets, as the queryset will override the instance you set.

Answer 2

An alternative approach:

class FormSetWithInstances(BaseFormSet):

    def get_form_kwargs(self, index):
        kwargs = super(FormSetWithInstances, self).get_form_kwargs(index)
        instances = kwargs.pop('instances')
        try:
            kwargs.update({'instance': instances[index]})
        except IndexError:
            pass
        return kwargs

Then, when creating instances of FormSetWithInstances, you pass in a list of instances as a form kwarg:

form_set = FormSetWithInstances(form_kwargs={'instances': [...]})

I personally prefer this method because it takes advantage of existing class infrastructure instead of defining custom class members in an overridden __init__(). Also, it's in the docs.

Answer 3

I'm not aware of an easy way to pass a list of instances as you are trying to do. Here's a couple of options that might work depending on your use case.

You can provide initial data for the model formset. This should be a list of dictionaries, not model instances:

initial = [{'name': 'a'}, {'name': 'b'}]
formset = MyFormSet(
    queryset=MyModel.objects.none(),
    initial=initial,
)

Note that I have set the queryset to an empty queryset. If you didn't do this, then the formset would display existing instances, and the initial data would be used for new instances.

If you have initial values for fields that you do not wish to include in the form, then you could be able to set those values when you [save the formset].

instances = formset.save(commit=False)
names = ['a', 'b']
for instance, name in zip(instances, names):
    instance.name = name
    instance.save()