def __init__(self, *args, **kwargs):
    systemid = kwargs.pop('systemid')
super(BackupForm, self).__init__(*args, **kwargs)
self.fields['units'] = forms.ModelMultipleChoiceField(required=False,
    queryset=Unit.objects.filter(system__id=systemid), widget=forms.
    SelectMultiple(attrs={'title': _('Add unit')}))


class Meta:
    model = Backup
exclude = 'system',
How to pass initial parameter to django's ModelForm instance?