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#1 Jan. 13, 2011 23:05:54

Aryeh L.
Registered: 2009-11-02
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Custom Model Select in Model Formset

I have models that look something like this:

class ProductGroup(models.Model):
name = models.CharField(max_length=10, primary_key=True)
def __unicode__(self): return self.name

class ProductRun(models.Model):
date = models.DateField(primary_key=True)
def __unicode__(self): return self.date.isoformat()

class CatalogItem(models.Model):
cid = models.CharField(max_length=25, primary_key=True)
group = models.ForeignKey(ProductGroup)
run = models.ForeignKey(ProductRun)
pnumber = models.IntegerField()
def __unicode__(self): return self.cid
class Meta:
unique_together = ('group', 'run', 'pnumber')

class Transaction(models.Model):
timestamp = models.DateTimeField()
user = models.ForeignKey(User)
item = models.ForeignKey(CatalogItem)
quantity = models.IntegerField()
price = models.FloatField()

Let's say there are about 10 ProductGroups and 10-20 relevant
ProductRuns at any given time. Each group has 20-200 distinct
product numbers (pnumber), so there are at least a few thousand

I am working on formsets for the Transaction model. Instead of a
single select menu with the several thousand CatalogItems for the
ForeignKey field, I want to substitute three drop-down menus, for
group, run, and pnumber, which uniquely identify the CatalogItem.
I'd also like to limit the choices in the second two drop-downs to
those runs and pnumbers which are available for the currently
selected product group (I can update them via AJAX if the user
changes the product group, but it's important that the initial page
load as described without relying on AJAX).

What's the best way to do this?

My first approach was to exclude the item foreign key field from the
form, add the substitute dropdowns by overriding the add_fields
method of the formset, and then extract the data and populate the
fields manually on the model instances before saving them. It's
straightforward and pretty simple, but it's not very reusable and I
don't think it is the right way to do this.

My second approach was to create a new field which inherits both
MultiValueField and ModelChoiceField, and a corresponding
MultiWidget subclass. This seems like the right approach. As
Malcolm put it in an earlier discussion, "the 'smarts' of a
field lie in the Field class."

The problem I'm having is when/where to fetch the lists of choices
from the db. The code I have now does it in the Field's __init__,
but that means I have to know which ProductGroup I'm dealing with
before I can create the Form class, since I have to instantiate the
Field when I define the form. So I have my own little factory
function which I call at the last minute from my view--after I know
what CatalogItems I have and which product group they're in--to
create form/formset classes and instantiate them. It works, but I
wonder if there's a better way.

Another problem is that my implementation limits the entire formset
to transactions relating to (CatalogItems from) a single

A third possibility I'm entertaining is to put it all in the Widget
class. Once I have the related model instance, or the cid, or
whatever the widget is given, I can get the ProductGroup and
construct the drop-downs. This would solve the issues with my
second approach, but doesn't seem like the right approach. It seems
to me that Widgets are meant to render to HTML (ibid.), not hit the
db. Also, unless I do some caching, I'd end up hitting the database
a few times to reconstruct the select choices for each such field on
the page.

Thanks for reading; comments and suggestions welcome.

-Aryeh Leib

http://tinyurl.com/djangoformsdesign(It seems the OP was trying to do something similar)

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