Chained autocomplete_fields in contrib.admin

[Roman]

Sometimes querysets for autocomplete_fields should be dependent and selection should be chained.

Imagine there is a model Address with references to Country and City models

class Country(models.Model):
    id = models.AutoField(primary_key=True)
    name = models.CharField(max_length=100)


class City(models.Model):
    id = models.AutoField(primary_key=True)
    country = models.ForeignKey(Country, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)


class Address(models.Model):
    id = models.AutoField(primary_key=True)
    country = models.ForeignKey(Country, on_delete=models.CASCADE)
    city = models.ForeignKey(City, on_delete=models.CASCADE, null=True, blank=True)

Also imagine admin form displays a Country drop-down with the list of all countries and a City drop-downwith. A user reasonably expects City drop-down to contain the list of all cities within previously selected country, but not all cities in the entire world.

Unfortunately, when current version of Django calls /admin/app/model/autocomplete no information about current selection in other drop-downs is sent, so there is no way to select cities of a specific country for a new unsaved model. So overriding "def get_search_results(self, request, queryset, search_term)" gives nothing, as required information is not available.

However, a small change to autocomplete.js allowed me to easily implement what I need.

'use strict';
{
    const $ = django.jQuery;
    const init = function($element, options) {
        const settings = $.extend({
            ajax: {
                data: function(params) {
                    var result = {
                        'term': params.term,
                        'page': params.page
                    };

                    $('.admin-autocomplete').each(function(index) {
                        result['selected_' + $(this).attr('name')] = $(this).val(); // Pass selections of all other select2 with request.
                    });

                    return result;
                }
            }
        }, options);
        $element.select2(settings);
    };
...
// the rest is not changed

Now, in "def get_search_results(self, request, queryset, search_term)" I can extract all required information from "request.GET"

I am no sure neither about if all autocomplete selects should be included, if all selects should be included. Probably it would be better to introduce some kind of opt-in configuration. Also, I am not sure about "selected_" prefix. I understand that this is not a generic code which should be included in the next version of Django. However, this small change works pretty fine for me, allowing to build much more friendly admin forms. I am ready to present pull request, if maintainers will be so kind to comment on idea in general and point at issues I have missed.