Limiting Django ModelChoiceField queryset to selected items

Question

Here is what I've been struggling for a day...

I have a Message model in which recipients is a ManyToManyField to the User model.

Then there is a form for composing messages. As there are thousands of users, it is not convenient to display the options in a multiple select widget in the form, which is the default behavior. Instead, using FcbkComplete jquery plugin, I made the recipients field look like an input field where the user types the recipients, and it WORKS.

But...

Although not visible on the form page, all the user list is rendered into the page in the select field, which is something I don't want for obvious reasons.

I tried overriding the ModelChoiceField's behavior manipulating validation and queryset, I played with the MultipleChoice widget, etc. But none of them worked and felt natural.

So, what is the (best) way to avoid having the whole list of options on the client side, but still be able to validate against a queryset?

Answer 1

I solve this by overriding the forms.ModelMultipleChoiceField's default widget. The new widget returns only the selected fields, not the entire list of options:

class SelectMultipleUserWidget(forms.SelectMultiple):
    def render_options(self, choices, selected_choices):
        choices = [c for c in self.choices if str(c[0]) in selected_choices]
        self.choices = choices
        return super(SelectMultipleUserWidget, 
                     self).render_options([], selected_choices)

class ComposeForm(forms.Form):
    recipients = forms.ModelMultipleChoiceField(queryset=User.objects.all(),
                                                widget=SelectMultipleUserWidget)
    ...