Django: MultiValueField and MultiWidget

Question

Django's documentation doesn't do a very thorough job of explaining how to use MultiValueField and MultiWidget. I've tried dissecting the one implementation and haven't had good results. Would someone mind giving me a quick pointer in the right direction?

My example:

widgets.py

from django import forms

class TestMultiWidget(forms.MultiWidget):

    def __init__(self, attrs=None):
        widgets = (
            forms.TextInput(attrs=attrs),
            forms.TextInput(attrs=attrs),
        )
        super(TestMultiWidget, self).__init__(widgets, attrs)

    def decompress(self, value):
        if value:
            return value.split(':::')[0:2]
        return ['', '']

fields.py

from django import forms
from widgets import TestMultiWidget

class TestMultiField(forms.MultiValueField):
    widget = TestMultiWidget

    def __init__(self, *args, **kwargs):
        fields = (
            forms.CharField(),
            forms.CharField(),
        )
        super(TestMultiField, self).__init__(fields, *args, **kwargs)

    def compress(self, data_list):
        if data_list:
            return ':::'.join(data_list)
        return ''

models.py

from django.db import models
from util.fields import TestMultiField

class Test(models.Model):
    a = models.CharField(max_length=128)
    b = TestMultiField()
    c = models.CharField(max_length=128)

admin.py

from django.contrib import admin
from models import Test
admin.site.register(Test)

And the resulting admin.

Anybody have a clue what's happening here? My guess is that there's some unintended exception suppression happening, but I haven't been able to locate the source.

Thanks!

Answer 1

Please notice that django.forms.MultiValueField is a form field and not a model field (like django.db.models.CharField). Therefore, it is not treated as a model field in your Test model, and was not created in your database. (You can check it with ./manage.py sqlall myapp).

Change your models.py to:

from django.db import models
from fields import TestMultiField

class TestMultiModelField(models.Field):

    def formfield(self, **kwargs):
        defaults = {'form_class': TestMultiField}
        defaults.update(kwargs)
        return super(TestMultiModelField, self).formfield(**defaults)

    def get_internal_type(self):
        return 'TextField'        

class Test(models.Model):
    a = models.CharField(max_length=128)
    b = TestMultiModelField()
    c = models.CharField(max_length=128)      

drop your table (on linux/mac: ./manage.py sqlclear myapp | ./manage.py dbshell) and syncdb to create your table, this time with column b. Check your admin now.

Explanation: To create a custom model field, follow this: https://docs.djangoproject.com/en/dev/howto/custom-model-fields/

The set the model field's matching form field, the formfield method was used.

(BTW, The "correct" way to design the model field is probably a bit different, using to_python and get_prep_value)