I find myself in a conundrum with related models and related formsets. I'll
try and simplify a rather complex set of relations to illustrate the bare
bones gist of my issue. Imagine the standard Djnago docs example:
class Musician(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
instrument = models.CharField(max_length=100)
class Album(models.Model):
artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
name = models.CharField(max_length=100)
release_date = models.DateField()
num_stars = models.IntegerField()
Now imagine I have a form in which I can create (or update, same issues
arise) a Musician and their Albums. I have a generic CreateView of Musician
and on it a formset of Albums. You can enter a Musician's name and data and
their albums and submit and it creates the objects in the database. All
this I have working fine. Can even us JS to dynamically alter the formset
and allow submission of any number of albums, and updated (with an
UpdateView) of such record families.
Further, note that in Musician a field is created, "album_set" that is the
set of Albums associated with that Musician.
Now I can put a clean() method in Musician, and it's works really
charmingly. If I raise a ValidattionError in clean() the form re-renders
with the message displayed (it's available in the context item "messages").
I love this it's very slick and easy.
Now let's say I have a criterion to enforce, say "A musician must have at
least one album."
I'd like to test for that and use this neat feedback mechanism to tell the
user. Alas when I get to clean() in Musician, the field album_set is None
as the Albums have not yet been cleaned and saved.
In my CreateView I can override is_valid() to do some checks as well, but I
have the same problem, none of the objects exist yet so I can't check of
integrity of the objects,.
I can override form_valid() and in that I save the submitted Musician and
Albums explicitly (this all works a dream) and after that can check for
integrity (that the Musician has at least 1 album), but I can't at this
point raise a ValidationError anymore to bounce back to the form with a
message. Validation is done with in is_valid and in form_valid() is a
little late to be doing it.
The Catch 22 I see is that to validate the relationships I need the objects
to exist and to save the objects I want the relationships validated.
Ouch. A conundrum.
I can imagine saving them all in a transaction and bailing if something is
awry (rolling back), all not a conceptual issue, but again in form_valid()
is a tad late, and in is_valid() - which calls clean() - the objects don't
exist yet.
Conclusion: I have to validate the request not the objects. But that is not
DRY. The code that parses a request and turns it into objects is inside of
Django. I don't want to repeat the effort. I want to hook into it at the
appropriate place to:
1) Validate the relationships between submitted models (in one HTML form,
multiple Django forms)
2) Do it at a time that allows easy fallback on the lovely message system.
And all I've concluded is form_valid() seems too late, is_valid() seems too
early but I see nothing between them. Is there place to hook into the post
processing that is most appropriate for this kind of need?
Please don't dwell on the specifics of the example, I actually have richer
more complex relationships with more formsets than that all working fine I
can create, update, list and work with it. All charming. I can validate
intra-model fields in clean() easily and that works a dream. But I'm stuck
on how to validate relationships, when and where that is best done.
Regards,
Bernd.
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