referenced_catalog = forms.ModelChoiceField(
    queryset=Inventory_unit_catalog.objects.all().filter(parent_business_unit_context_id=user_context_id),

def __init__(self, *args, **kwargs):
        self.user_context_id = kwargs.pop('user_context_id', None)
        super(InventoryStockAddForm, self).__init__(*args, **kwargs)

SessionStore = import_module(settings.SESSION_ENGINE).SessionStore
s = SessionStore()
user_context_id = s['user_context_id']