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

Question

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.

Answer 1

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

Update: ForeignKey.limit_choices_to allows one 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.

Answer 2

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

Answer 3

The new "right" way of doing this, at least since Django 1.1 is by overriding the AdminModel.formfield_for_foreignkey(self, db_field, request, **kwargs).

See http://docs.djangoproject.com/en/1.2/ref/contrib/admin/#django.contrib.admin.ModelAdmin.formfield_for_foreignkey

For those who don't want to follow the link below is an example function that is close for the above questions models.

class MyModelAdmin(admin.ModelAdmin):
    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == "favoritechild":
            kwargs["queryset"] = Child.objects.filter(myparent=request.object_id)
        return super(MyModelAdmin, self).formfield_for_manytomany(db_field, request, **kwargs)

I'm only not sure about how to get the current object that is being edited. I expect it is actually on the self somewhere but I'm not sure.

Answer 4

This isn't how django works. You would only create the relation going one way.

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

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

And if you were trying to access the children from the parent you would do parent_object.child_set.all(). If you set a related_name in the myparent field, then that is what you would refer to it as. Ex: related_name='children', then you would do parent_object.children.all()

Read the docs http://docs.djangoproject.com/en/dev/topics/db/models/#many-to-one-relationships for more.

Answer 5

If you only need the limitations in the Django admin interface, this might work. I based it on this answer from another forum - although it's for ManyToMany relationships, you should be able to replace formfield_for_foreignkey for it to work. In admin.py:

class ParentAdmin(admin.ModelAdmin):
    def get_form(self, request, obj=None, **kwargs):
        self.instance = obj
        return super(ParentAdmin, self).get_form(request, obj=obj, **kwargs)

    def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
        if db_field.name == 'favoritechild' and self.instance:       
            kwargs['queryset'] = Child.objects.filter(myparent=self.instance.pk)
        return super(ChildAdmin, self).formfield_for_foreignkey(db_field, request=request, **kwargs)

Answer 6

I'm trying to do something similar. It seems like everyone saying 'you should only have a foreign key one way' has maybe misunderstood what you're trying do.

It's a shame the limit_choices_to={"myparent": "self"} you wanted to do doesn't work... that would have been clean and simple. Unfortunately the 'self' doesn't get evaluated and goes through as a plain string.

I thought maybe I could do:

class MyModel(models.Model):
    def _get_self_pk(self):
        return self.pk
    favourite = models.ForeignKey(limit_choices_to={'myparent__pk':_get_self_pk})

But alas that gives an error because the function doesn't get passed a self arg :(

It seems like the only way is to put the logic into all the forms that use this model (ie pass a queryset in to the choices for your formfield). Which is easily done, but it'd be more DRY to have this at the model level. Your overriding the save method of the model seems a good way to prevent invalid choices getting through.

Update
See my later answer for another way https://stackoverflow.com/a/3753916/202168

Answer 7

@Ber: I have added validation to the model similar to this

class Parent(models.Model):
  name = models.CharField(max_length=255)
  favoritechild = models.ForeignKey("Child", blank=True, null=True)
  def save(self, force_insert=False, force_update=False):
    if self.favoritechild is not None and self.favoritechild.myparent.id != self.id:
      raise Exception("You must select one of your own children as your favorite")
    super(Parent, self).save(force_insert, force_update)

which works exactly how I want, but it would be really nice if this validation could restrict choices in the dropdown in the admin interface rather than validating after the choice.

Answer 8

Do you want to restrict the choices available in the admin interface when creating/editing a model instance?

One way to do this is validation of the model. This lets you raise an error in the admin interface if the foreign field is not the right choice.

Of course, Eric's answer is correct: You only really need one foreign key, from child to parent here.

Answer 9

An alternative approach would be not to have 'favouritechild' fk as a field on the Parent model.

Instead you could have an is_favourite boolean field on the Child.

This may help: https://github.com/anentropic/django-exclusivebooleanfield

That way you'd sidestep the whole problem of ensuring Children could only be made the favourite of the Parent they belong to.

The view code would be slightly different but the filtering logic would be straightforward.

In the admin you could even have an inline for Child models that exposed the is_favourite checkbox (if you only have a few children per parent) otherwise the admin would have to be done from the Child's side.

Answer 10

A much simpler variation of @s29's answer:

Instead of customising the form, You can simply restrict the choices available in form field from your view:

what worked for me was: in forms.py:

class AddIncomingPaymentForm(forms.ModelForm):
    class Meta: 
        model = IncomingPayment
        fields = ('description', 'amount', 'income_source', 'income_category', 'bank_account')

in views.py:

def addIncomingPayment(request):
    form = AddIncomingPaymentForm()
    form.fields['bank_account'].queryset = BankAccount.objects.filter(profile=request.user.profile)

Answer 11

from django.contrib import admin
from sopin.menus.models import Restaurant, DishType

class ObjInline(admin.TabularInline):
    def __init__(self, parent_model, admin_site, obj=None):
        self.obj = obj
        super(ObjInline, self).__init__(parent_model, admin_site)

class ObjAdmin(admin.ModelAdmin):

    def get_inline_instances(self, request, obj=None):
        inline_instances = []
        for inline_class in self.inlines:
            inline = inline_class(self.model, self.admin_site, obj)
            if request:
                if not (inline.has_add_permission(request) or
                        inline.has_change_permission(request, obj) or
                        inline.has_delete_permission(request, obj)):
                    continue
                if not inline.has_add_permission(request):
                    inline.max_num = 0
            inline_instances.append(inline)

        return inline_instances



class DishTypeInline(ObjInline):
    model = DishType

    def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
        field = super(DishTypeInline, self).formfield_for_foreignkey(db_field, request, **kwargs)
        if db_field.name == 'dishtype':
            if self.obj is not None:
                field.queryset = field.queryset.filter(restaurant__exact = self.obj)  
            else:
                field.queryset = field.queryset.none()

        return field

class RestaurantAdmin(ObjAdmin):
    inlines = [
        DishTypeInline
    ]