ModelChoiceField and dynamic Querysets

[mwebs]

With the ModelChoiceFiled you can represent relationships between
models.

So my form looks something like this:

class MyForm(forms.Form):
name = forms.CharField()
trials = forms.ModelChoiceField(...)

According to the documentation I have to pass the queryset to the
modelchoicefield.
But how can I do this:

e.g. I want to pass somethiung like this:

queryset = MyModel.objects.filter(a_fk = .., b_fk = null).exclude(...)

How do I pass this to the trials formField? The filter lookup also
takes some variables into account and in my forms.py I dont see a way
to realize this, as I can not pass any arguments.

How can I pass a dynamic generated queryset?
Thanks

[mwebs]

This solved the problem:

http://groups.google.com/group/django-users/browse_thread/thread/63c2058c7892f865