Programmatically specifying Django model attributes
- by mojbro
Hi!
I would like to add attributes to a Django models programmatically, at run time. For instance, lets say I have a Car model class and want to add one price attribute (database column) per currency, given a list of currencies.
What is the best way to do this?
I had an approach that I thought would work, but it didn't exactly. This is how I tried doing it, using the car example above:
from django.db import models
class Car(models.Model):
name = models.CharField(max_length=50)
currencies = ['EUR', 'USD']
for currency in currencies:
Car.add_to_class('price_%s' % currency.lower(), models.IntegerField())
This does seem to work pretty well at first sight:
$ ./manage.py syncdb
Creating table shop_car
$ ./manage.py dbshell
shop=# \d shop_car
Table "public.shop_car"
Column | Type | Modifiers
-----------+-----------------------+-------------------------------------------------------
id | integer | not null default nextval('shop_car_id_seq'::regclass)
name | character varying(50) | not null
price_eur | integer | not null
price_usd | integer | not null
Indexes:
"shop_car_pkey" PRIMARY KEY, btree (id)
But when I try to create a new Car, it doesn't really work anymore:
>>> from shop.models import Car
>>> mycar = Car(name='VW Jetta', price_eur=100, price_usd=130)
>>> mycar
<Car: Car object>
>>> mycar.save()
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/base.py", line 410, in save
self.save_base(force_insert=force_insert, force_update=force_update)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/base.py", line 495, in save_base
result = manager._insert(values, return_id=update_pk)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/manager.py", line 177, in _insert
return insert_query(self.model, values, **kwargs)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/query.py", line 1087, in insert_query
return query.execute_sql(return_id)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/sql/subqueries.py", line 320, in execute_sql
cursor = super(InsertQuery, self).execute_sql(None)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/models/sql/query.py", line 2369, in execute_sql
cursor.execute(sql, params)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/django/db/backends/util.py", line 19, in execute
return self.cursor.execute(sql, params)
ProgrammingError: column "price_eur" specified more than once
LINE 1: ...NTO "shop_car" ("name", "price_eur", "price_usd", "price_eur...
^
