How do I restrict foreign keys choices to related objects only in django

I have a two way foreign relation similar to the following

class Parent(models.Model):
  name = models.CharField(max_length=255)
  favoritechild = models.ForeignKey("Child", blank=True, null=True)

class Child(models.Model):
  name = models.CharField(max_length=255)
  myparent = models.ForeignKey(Parent)

How do I restrict the choices for Parent.favoritechild to only children whose parent is itself? I tried

class Parent(models.Model):
  name = models.CharField(max_length=255)
  favoritechild = models.ForeignKey("Child", blank=True, null=True, limit_choices_to = {"myparent": "self"})

but that causes the admin interface to not list any children.

I just came across ForeignKey.limit_choices_to in the Django docs. Not sure yet how this works, but it might just be the right thing here.

Update: ForeignKey.limit_choices_to allows to specify either a constant, a callable or a Q object to restrict the allowable choices for the key. A constant obviously is of no use here, since it knows nothing about the objects involved.

Using a callable (function or class method or any callable object) seems more promising. However, the problem of how to access the necessary information from the HttpRequest object remains. Using thread local storage may be a solution.

2. Update: Here is what has worked for me:

I created a middleware as described in the link above. It extracts one or more arguments from the request's GET part, such as "product=1", and stores this information in the thread locals.

Next there is a class method in the model that reads the thread local variable and returns a list of ids to limit the choice of a foreign key field.

@classmethod
def _product_list(cls):
    """
    return a list containing the one product_id contained in the request URL,
    or a query containing all valid product_ids if not id present in URL

    used to limit the choice of foreign key object to those related to the current product
    """
    id = threadlocals.get_current_product()
    if id is not None:
        return [id]
    else:
        return Product.objects.all().values('pk').query

It is important to return a query containing all possible ids if none was selected so that the normal admin pages work ok.

The foreign key field is then declared as:

product = models.ForeignKey(
    Product,
    limit_choices_to={
        id__in=BaseModel._product_list,
    },
)

The catch is that you have to provide the information to restrict the choices via the request. I don't see a way to access "self" here.

The 'right' way to do it is to use a custom form. From there, you can access self.instance, which is the current object. Example --

from django import forms
from django.contrib import admin 
from models import *

class SupplierAdminForm(forms.ModelForm):
    class Meta:
        model = Supplier
        fields = "__all__" # for Django 1.8+


    def __init__(self, *args, **kwargs):
        super(SupplierAdminForm, self).__init__(*args, **kwargs)
        if self.instance:
            self.fields['cat'].queryset = Cat.objects.filter(supplier=self.instance)

class SupplierAdmin(admin.ModelAdmin):
    form = SupplierAdminForm