Mike, Anssi, Thank you for your replies. I will give this a shot and see if I can get it to work.
Sam On Thursday, May 17, 2012 9:59:40 PM UTC-7, akaariai wrote: > > On May 18, 5:25 am, oneroler <samuel.ha...@gmail.com> wrote: > > Thanks Mike, that is what I was originally planning to do but realized > > there would be situations where that wouldn't do exactly what I wanted. > > For example, if there is a business that only has the strategy > 'wholesale' > > assigned, using ForeignKey would still allow me to assign a different > > strategy to a division. I was hoping to find a solution where the > strategy > > for a division is constrained by the strategies assigned to its > respective > > business. > > Django doesn't make this particularly easy. You should create the > constraint in the database by using custom SQL, and then constraint > the assignable objects in Python in your view code. The Strategy and > Business models will create three tables into the database, one of the > tables is the many-to-many table. The m2m table's structure should be > something like this (check manage.py sqlall output): > > create table business_strategy( > id serial primary key, > business_id integer references strategy(id), > strategy id integer references business(id), > unique (business_id, strategy_id) > ) > > Now, what you need to do is create a foreign key pointing to > business_id, strategy_id for your division table. The model should > look like this: > > class Division(models.Model): > business = models.ForeignKey(Business) > name = models.CharField() > strategy = models.ForeignKey(Strategy) > > this creates a table ( > id serial primary key, > business_id integer references business(id), > name varchar(n), > strategy_id integer references strategy(id), > ) > > You will need to drop the strategy_id -> strategy(id) foreign key by > hand and add a new one: business_id, strategy_id -> > business_strategy(business_id, strategy_id). Check your database > vendors docs for details how to do this. You will want this constraint > to be deferred if available on your DB. > > So, now you should have correct constraint in the database. You will > still have some problems: changing division's business changes the set > of available strategies. On UI side you will probably need to create > an AJAX call on business changed to fetch the available strategies. In > forms code you will need to do something like this: > > class DivisionForm(forms.ModelForm): > class Meta: > model = Division > def __init__(self, **kwargs): > # Note, intentionally restrict the forms usage to only > kwargs. > super(DivisionForm, self).__init__(**kwargs) > business_id = None > if 'initial' in kwargs: > business_id = kwargs['initial'].business_id > if 'data' in kwargs and 'business_id' in kwargs['data']: > business_id = kwargs['data']['business_id'] > self.fields['strategy'].queryset = > Strategy.objects.filter(business_set=business_id) > > So, the restriction should be doable both in DB and Django, but isn't > trivial to do. The above is pseudo-code style, so the example code > will likely not work by copy-pasting... > > - Anssi -- You received this message because you are subscribed to the Google Groups "Django users" group. To view this discussion on the web visit https://groups.google.com/d/msg/django-users/-/JdPApZhO6x8J. To post to this group, send email to django-users@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
