Django ForeignKey limit_choices_to a different ForeignKey id

Question

I'm trying to limit Django Admin choices of a ForeignKey using limit_choices_to, but I can't figure out how to do it properly.

This code does what I want if the category id is 16, but I can't figure out how to use the current category id rather than hard-coding it.

class MovieCategory(models.Model):    
    category = models.ForeignKey(Category)
    movie = models.ForeignKey(Movie)
    prefix = models.ForeignKey('Prefix', limit_choices_to={'category_id': '16'},
                               blank=True, null=True)
    number = models.DecimalField(verbose_name='Movie Number', max_digits=2,
                                 blank=True, null=True, decimal_places=0)

Is it possible to refer to the id of the category ForeignKey somehow?

Answer 1

After hours of reading semi related questions I finally figured this out.

You can't self reference a Model the way I was trying to do so there is no way to make django act the way I wanted using limit_choices_to because it can't find the id of a different ForeignKey in the same model.

This can apparently be done if you change the way django works, but a simpler way to solve this was to make changes to admin.py instead.

Here is what this looks like in my models.py now:

# models.py
class MovieCategory(models.Model):    
    category = models.ForeignKey(Category)
    movie = models.ForeignKey(Movie)
    prefix = models.ForeignKey('Prefix', blank=True, null=True)
    number = models.DecimalField(verbose_name='Movie Number', max_digits=2,
                                 blank=True, null=True, decimal_places=0)

I simply removed limit_choices_to entirely. I found a similar problem here with the solution posted by Kyle Duncan. The difference though is that this uses ManyToMany and not ForeignKey. That means I had to remove filter_horizontal = ('prefix',) under my class MovieCategoryAdmin(admin.ModelAdmin): as that is only for ManyToMany fields.

In admin.py I had to add from django import forms at the top to create a form. This is how the form looks:

class MovieCategoryForm(forms.ModelForm):

    class Meta:
        model = MovieCategory
        fields = ['prefix']

    def __init__(self, *args, **kwargs):
        super(MovieCategoryForm, self).__init__(*args, **kwargs)
        self.fields['prefix'].queryset = Prefix.objects.filter(
                                        category_id=self.instance.category.id)

And my AdminModel:

class MovieCategoryAdmin(admin.ModelAdmin):
    """
    Admin Class for 'Movie Category'.
    """
    fieldsets = [
        ('Category',      {'fields': ['category']}),
        ('Movie',         {'fields': ['movie']}),
        ('Prefix',        {'fields': ['prefix']}),
        ('Number',        {'fields': ['number']}),
    ]
    list_display = ('category', 'movie', 'prefix', 'number')
    search_fields = ['category__category_name', 'movie__title', 'prefix__prefix']
    form = MovieCategoryForm

This is exactly how Kyle describes it in his answer, except I had to add fields = ['prefix'] to the Form or it wouldn't run. If you follow his steps and remember to remove filter_horizontal and add the fields you're using it should work.

Edit: This solution works fine when editing, but not when creating a new entry because it can't search for the category id when one doesn't exits. I am trying to figure out how to solve this.