Filter a Django form select element based on a previously selected element

Question

Let\'s consider the following models

models.py

Class Brand(models.Model):
    company_name = models.CharField(max_length=100)


class CarModel(models.Model         


        

Answer 1

class Country(models.Model):
   country_name=models.CharField(max_length=10, blank=True, null=True)

class State(models.Model):
   state_name=models.CharField(max_length=10, blank=True, null=True)

class MyCustomModal(models.Model):
   country = models.ForeignKey(Country, on_delete=models.CASCADE, null=True, blank=True)
   state = models.ForeignKey(State, on_delete=models.CASCADE, null=True, blank=True)

Here is my Form

class MyCustomForm(forms.ModelForm):
    class Meta:
        model = MyCustomModal
        fields = [
            'country',
            'state',           
        ]        

    def __init__(self, *args, **kwargs):
        super(MyCustomForm, self).__init__(*args, **kwargs)        
        self.fields['country'] = forms.ChoiceField(choices=[('1','india'),('2','US')])
        self.fields['state'].queryset = State.objects.filter(pk=2)

Answer 2

After hours and hours of research, without success, I decided to try to solve on my own. The solution that I found maybe don't be the best or the more elegant, but is working. (For download full Django project, click on this repo => https://github.com/Sidon/djfkf/.)

---

models.py

from django.db import models

class Brand(models.Model):
    company_name = models.CharField(max_length=100)

    def __str__(self):
        return self.company_name


class Car(models.Model):
    brand = models.ForeignKey(Brand)
    name = models.CharField(max_length=100)

    def brand_name(self):
        return self.brand.company_name

    def __str__(self):
        return self.name


class Fleet(models.Model):
    car = models.ForeignKey(Car)
    description = models.CharField(max_length=100)

    def car_name(self):
        return self.car.name

    def brand(self):
        return self.car.brand.company_name

    def __str__(self):
        return self.description

The goal is to register cars on the fleet. The only fields that are will be recorded: Car (foreign key) and description. On the form, there will be one select element for brands that will work just only as a helper for to filter the car's combo box.

---

forms.py

import json
from django import forms
from .models import *

class RegCarForm(forms.ModelForm):

    dcars = {}
    list_cars = []
    for car in Car.objects.all():
        if car.brand.company_name in dcars:
            dcars[car.brand.company_name].append(car.name)
        else:
            dcars[car.brand.company_name] = [car.name]
        list_cars.append((car.name,car.name))

    brands = [str(brand) for brand in Brand.objects.all()]

    brand_select = forms.ChoiceField(choices=([(brand, brand) for brand in brands]))
    car_select = forms.ChoiceField(choices=(list_cars))

    brands = json.dumps(brands)
    cars = json.dumps(dcars)

    class Meta:
        model = Fleet
        fields = ('brand_select', 'car_select', 'description',)

RegCarForm is a form for register cars, there are three fields: brand_select, car_select, and description. In addition, I defined two JSON attributes: 1) a dictionary whose keys are brands (strings) and values are lists of respective's cars and 2) A list of strings that represent the brands. Those two attributes will work as helpers for JS functions.

---

views.py

from django.shortcuts import render
from .forms import RegCarForm
from .models import *

def regcar(request):
    if request.method == 'POST':
        car_form = RegCarForm(data=request.POST)

        if car_form.is_valid():
            cdata = car_form.cleaned_data.get
            car_selected = Car.objects.filter(name=cdata('car_select'))
            reg1 = Fleet(car_id=car_selected[0].id, description=cdata('description'))
            reg1.save()
        else:
            print ('Invalid')

    else:
        car_form = RegCarForm()
    return render(request, 'core/regcar.html', {'car_form': car_form})

The view is practically auto-explanatory. Assigns the Form to the car_form variable, render the template core/regcar.html and, after Post, make the validation of the form and save the data.

---

regcar.html (template django)

{% extends "base.html" %}

{% block head %}
{% endblock %}

{% block content %}
    <h1>Registering cars on the fleet. <br />(Populate one drop down based on selection in another)</h1>
    <p>Change the contents of drop down Car based on the selection in dropdown Brand, using Django-forms + Javascritp</p>
    <div class="select-style">
        <form action="." method="post">
            {% csrf_token %}
            {{ car_form.as_p }}
            <p><input type="submit" value="Register a car"></p>
        </form>
    </div>
{% endblock %}

{% block js %}
    {% include "js1.html" %}
{% endblock %}

The template only just renders the form and load the script JS. Nothing else.

---

Finally, the js script, that makes the hard work.

{% block js %}
    <script language="javascript">
        $('#id_brand_select').change(function() {populateCar(this)});
        $('#id_description').addClass('descriptions');
        cars = {{ car_form.cars | safe }}
        brands = {{ car_form.brands | safe}};
        populateBrand();
        $("#id_car_select").empty();
        $("#id_car_select").append('<option value="" disabled selected>First select a brand</option>');
        function populateBrand() {
            $('#id_brand_select').empty();
            $("#id_brand_select").append('<option value="" disabled selected>Select your option</option>');
            $.each(brands, function(v) {
                $('#id_brand_select')
                    .append($("<option></option>")
                    .attr("value", brands[v])
                    .text(brands[v]));
            });
        }

        function populateCar(event) {
            brand = $("#id_brand_select option:selected").text();
            $("#id_car_select").empty();
            $("#id_car_select").append('<option value="" disabled selected>Select your option</option>');
            for (let [b, bcars] of Object.entries(cars)) {
                if (b == brand) {
                    //alert(b);
                    for (car in bcars) {
                        $('#id_car_select')
                            .append($("<option></option>")
                                .attr("value", bcars[car])
                                .text(bcars[car]));
                    }
                }
            }
        }
    </script>
{% endblock %}

When the document is loaded, this script assigns the change event of brand_select (combo for selection of brand) to the function poplulateCar, assign the form's JASON attributes (cars and brands) to a JS variables and call the populateBrand function.

Links:

Full project in Django:
https://github.com/Sidon/djfkf/