Dynamic django form using kwargs

In views.py:

from django.views.generic import FormView

class ActivityView(FormView):

    def get_form_kwargs(self):
        kwargs = super(ActivityView, self).get_form_kwargs()
        kwargs['activity_id'] = self.kwargs.get('activity_id', None)
        return kwargs

In forms.py:

    class ActivityForm(ModelForm):

        def __init__(self, *args, **kwargs):

            self.activity_id = kwargs.pop('activity_id', None)
            super(ActivityForm, self).__init__(*args, **kwargs)

            self.fields['somefield'].queryset = Activity.objects.filter(id=self.activity_id)

Further info here