Django: accessing the model instance from within ModelAdmin?

Question

I've got a model for Orders in a webshop application, with an auto-incrementing primary key and a foreign key to itself, since orders can be split into multiple orders, but the relationship to the original order must be maintained.

class Order(models.Model):     ordernumber = models.AutoField(primary_key=True)     parent_order = models.ForeignKey('self', null=True, blank=True, related_name='child_orders')     # .. other fields not relevant here 

I've registered an OrderAdmin class for the admin site. For the detail view, I've included parent_order in the fieldsets attribute. Of course, by default this lists all the orders in a select box, but this is not the desired behaviour. Instead, for orders that don't have a parent order (i.e. have not been split from another order; parent_order is NULL/None), no orders should be displayed. For orders that have been split, this should only display the single parent order.

There's a rather new ModelAdmin method available, formfield_for_foreignkey, that seems perfect for this, since the queryset can be filtered inside it. Imagine we're looking at the detail view of order #11234, which has been split from order #11208. The code is below

def formfield_for_foreignkey(self, db_field, request, **kwargs):     if db_field.name == 'parent_order':         # kwargs["queryset"] = Order.objects.filter(child_orders__ordernumber__exact=11234)         return db_field.formfield(**kwargs)     return super(OrderAdmin, self).formfield_for_foreignkey(db_field, request, **kwargs) 

The commented row works when run in a Python shell, returning a single-item queryset containing order #11208 for #11234 and all other orders that may have been split from it.

Of course, we can't hard-code the order number there. We need a reference to the ordernumber field of the order instance whose detail page we're looking at. Like this:

kwargs["queryset"] = Order.objects.filter(child_orders__ordernumber__exact=?????) 

I've found no working way to replace ????? with a reference to the "current" Order instance, and I've dug pretty deep. self inside formfield_for_foreignkey refers to the ModelAdmin instance, and while that does have a model attribute, it's not the order model instance (it's a ModelBase reference; self.model() returns an instance, but its ordernumber is None).

One solution might be to pull the order number from request.path (/admin/orders/order/11234/), but that is really ugly. I really wish there is a better way.

Answer 1

I think you might need to approach this in a slightly different way - by modifying the ModelForm, rather than the admin class. Something like this:

class OrderForm(forms.ModelForm):      def __init__(self, *args, **kwargs):         super(OrderForm, self).__init__(*args, **kwargs)         self.fields['parent_order'].queryset = Order.objects.filter(             child_orders__ordernumber__exact=self.instance.pk)  class OrderAdmin(admin.ModelAdmin):     form = OrderForm 

Answer 2

I've modeled my inline class this way. It's a bit ugly on how it gets the parent form id to filter inline data, but it works. It filters units by company from the parent form.

The original concept is explained here http://www.stereoplex.com/blog/filtering-dropdown-lists-in-the-django-admin

class CompanyOccupationInline(admin.TabularInline):      model = Occupation     # max_num = 1     extra = 0     can_delete = False     formset = RequiredInlineFormSet      def formfield_for_dbfield(self, field, **kwargs):          if field.name == 'unit':             parent_company = self.get_object(kwargs['request'], Company)             units = Unit.objects.filter(company=parent_company)             return forms.ModelChoiceField(queryset=units)         return super(CompanyOccupationInline, self).formfield_for_dbfield(field, **kwargs)      def get_object(self, request, model):         object_id = resolve(request.path).args[0]         try:             object_id = int(object_id)         except ValueError:             return None         return model.objects.get(pk=object_id)