Django Admin: Many-to-Many listbox doesn't show up with a through parameter

Question

I have the following models:

class Message(models.Model):
    date = models.DateTimeField()
    user = models.ForeignKey(User)    
    thread = models.ForeignKey         


        

Answer 1

I learned a lot from @Fedor's answer, but some comments and cleanup may be still beneficial.

class ForumAdminForm(forms.ModelForm):
    messages = forms.ModelMultipleChoiceField(
                   queryset=Message.objects.all(),
                   widget=FilteredSelectMultiple('Message', False))


    # Technically, you don't need to manually set initial here for ForumAdminForm
    # However, you NEED to do the following for MessageAdminForm
    def __init__(self, *args, **kwargs):
        if 'instance' in kwargs:
            # a record is being changed. building initial
            initial = kwargs.setdefault('initial', {})
            initial['messages'] = [t.message.pk for t in kwargs['instance'].message_forum_set.all()]
        super(ForumAdminForm, self).__init__(*args, **kwargs)

    def save(self, commit=True):
        if not self.is_valid():
            raise HttpResponseForbidden
        instance = super(ForumAdminForm, self).save(self, commit)
        def save_m2m_with_through():
            messages = [t for t in self.cleaned_data['messages']
            old_memberships = instance.message_forum_set.all()
            for old in old_memberships:
                if old.message not in messages:
                    # and old membership is cleaned by the user
                    old.delete()
            for message in [x for x in messages not in map(lambda x: x.message, old_memberships)]:                   
                membership = Member_forum(message=messsage, forum=instance) 
                # You may have to initialize status, position and tag for your need
                membership.save()
        if commit:
            save_m2m_with_through()
        else:
            self.save_m2m = save_m2m_with_through
        return instance

    class Meta:
        model = Forum
        fields = {'name', 'messages')

There's one caveat: if you have another many-to-many relationship in the models (that is without through), super(ForumAdminForm, self).save(self, commit) will set self.save_m2m in case commit is False. However, calling this would cause an error, because this function also tries to save the many-to-many with through as well. You may need to save all other many-to-many relationship manually, or catch the exception, or else.