ChoiceField: where to initialize choices

Hello,

Considering this basic model:

******************************************************
*from django.contrib.auth.models import User
*
*class Meeting(models.Model):
*    chairmen = models.ManyToManyField(User,
related_name="meetings")
*    participants = models.ManyToManyField(User,
related_name="participants")
******************************************************

How could I build the form to create a new Meeting?
My problem is to initialize the MultipleChoiceField's
choices with the same list
of users for both chairmen and participants:

======================================================
=class MeetingForm(forms.Form):
=    chairmen =
forms.MultipleChoiceField(choices=[(u.id,u.name) for u in
User.objects.all()])
=    participants =
forms.MultipleChoiceField(choices=[(u.id,u.name) for u in
User.objects.all()])
======================================================

Not DRY enough to me, so I tried initializing choices in the
__init__():

======================================================
= (...)
= def __init__(self, *args, **kwargs):
=     super(MeetingForm, self).__init__(*args, **kwargs)
=     user_list = [(u.id,u.name) for u in
User.objects.all()]
=     self.fields['chairmen'].choices = user_list
=     self.fields['participants'].choices = user_list
======================================================

When displaying this (supposedly unbound) form, it complains
about validation
errors, with the luxury of not displaying the users in the
HTML output.
I could not find a proper answer to my problem in previous
topics on this list.

Any advice on my code would be much appreciated (I know I
could use
form_for_model, but I'm trying to learn newforms without its
black magic
shortcuts).

Kind regards.

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