form.save_m2m and intermediary models

[Delta20]

I'm getting the error "Cannot set values on a ManyToManyField which
specifies an intermediary model." when I try to call form.save_m2m()
and I would appreciate any tips as to how to work around this.

Here's some code that illustrates what I am doing:

---
# Using a basic form in a view function:

ticket = form.save(commit=False)
ticket.save()
form.save_m2m() # this does not work

---
# in models.py:

class System(models.Model):
# fields omitted for brevity

class Ticket(models.Model):
systems = models.ManyToManyField(System, through='SystemTicket')

def do_stuff(self):
# Do some stuff that has to happen after m2m

# custom m2m so we can act on the post_save signal
class SystemTicket(models.Model):
ticket = models.ForeignKey(Ticket)
system = models.ForeignKey(System)

def update_stuff(instance, **kwargs):
instance.ticket.do_stuff()
instance.ticket.save()

post_save.connect(update_stuff, sender=SystemTicket)

[Delta20]

I forgot to add: the full error message is:

"Cannot set values on a ManyToManyField which specifies an

intermediary model. Use SystemTicket's Manager instead."

I'm not sure how I'd use the manager in this particular case though.

[Malcolm Tredinnick]

On Thu, 2009-03-19 at 14:21 -0700, Delta20 wrote:
> I'm getting the error "Cannot set values on a ManyToManyField which
> specifies an intermediary model." when I try to call form.save_m2m()
> and I would appreciate any tips as to how to work around this.

The error is telling you exactly what the problem is. You can't do the
automatic assignment to many-to-many relations that use an intermediate
table. The reason being that the whole point of an intermediate table is
that it has *extra* information specified on it (otherwise you could
just use a normal ManyToManyField). So you need to create objects with
that extra information and save then explicitly.

Short version is you can't use save_m2m() in that case for precisely
this reason. You'll need to extract out the necessary data from the
form, and save it on the intermediate model.

Regards,
Malcolm

[Delta20]

On Mar 19, 6:50 pm, Malcolm Tredinnick <malc...@pointy-stick.com>
wrote:

> The error is telling you exactly what the problem is. You can't do the
> automatic assignment to many-to-many relations that use an intermediate
> table. The reason being that the whole point of an intermediate table is
> that it has *extra* information specified on it (otherwise you could
> just use a normal ManyToManyField). So you need to create objects with
> that extra information and save then explicitly.
>
> Short version is you can't use save_m2m() in that case for precisely
> this reason. You'll need to extract out the necessary data from the
> form, and save it on the intermediate model.

In this case, all I need is the post_save signal for when the m2m is
saved. The reason being is that I have Ticket.do_stuff in the example
above computes the value of a field in Ticket and the value depends on
what what Systems are related.

Is there a way I can achieve that without a custom intermediate model?
Manually invoking a method to do this doesn't work because that would
break in cases like modifying things in admin.