A case for values_list

Published on 2010-07-19 16:25:00+00:00
django   python   web  

Here's the Scenario: I have a model (lets call it Contact) with two Foreign Keys, one of which is related to User in Django's contrib.auth app. I need to build a form that lets me select an existing object, and a new user.

class ContactType(Model):  
    name = CharField(max_length=128)  
class Contact(Model):  
    user = ForeignKey(User)  
    contact_type = ForeignKey(ContactType)  
    # possibly more fields...

I need to select from existing models, so my first thought might be to build a form that uses two [ModelChoiceField](http://docs.djangoproject.com/en/1.2/ref/forms/fields/#django.forms.ModelChoiceField)'s. I also want to modify the way that my form displays each choice, so I could extend ModelChoiceField by overriding the label_from_instance method:

class UserModelChoiceField(ModelChoiceField):  
    def label\_from\_instance(self, obj):  
        return "%s (%s)"%(obj.get_full_name(), obj.username)  
class ContactModelChoiceField(ModelChoiceField):  
    def label\_from\_instance(self, obj):  
        return '%s (%s)' % (obj.type, obj.user.get_full_name())  
class CopyContactForm(forms.Form):  
    contact = ContactModelChoiceField(Contact.objects.all())  
    new_user = UserModelChoiceField(User.objects.all().order_by('first\_name', 'last\_name', 'username')) 

This actually provides a solution to my original problem, but it's not very efficient. Notice that both the UserModelChoiceField and the ContactModelChoiceField call methods on each object with the latter accessing a foreign key. In an app with 600 Users and 600 Contacts, this form would generate around 1200 queries!

There's actually a very efficient way to generate the same sort of form using [values\_list](http://docs.djangoproject.com/en/dev/topics/db/optimization/#use-queryset-values-and-values-list), especially, when you realize that the form really just needs to contain something like the following:

<option value="1">John Doe</option>  
<option value="2">Jane Doe</option>  

So a more efficient solution to my problem looks something like the code below, which yields two queries.

class CopyContactForm(forms.Form):  
    contact = forms.ChoiceField(choices=[(c[0], '%s (%s %s)'%(c[1],c[2],c[3])) \  
        for c in Contact.objects.values_list('id', 'type\_\_name', 'user\_\_first\_name', 'user\_\_last\_name')])  
    new_user = forms.ChoiceField(choices=[(u[0], '%s %s (%s)'%(u[1],u[2],u[3])) \  
        for u in User.objects.values_list('id', 'first\_name', 'last\_name', 'username')])