How to change the Django admin filter to use a dropdown instead of list that can also be searched?

I was struggling with the same problem some few weeks back. So this answer might be useful to some developers from the future.

I managed to solve the problem by writing a custom template.html

I have bundled the code in an amazing package now that does the same for you, here's the link.

Here's how you can implement a Searchable Dropdown in place of the default List:

1. Installation:

pip install django-admin-searchable-dropdown

This command will install the latest version of the package in your project.
Now, include the package in your project by adding admin_searchable_dropdown to your INSTALLED_APPS inside settings.py file.

2. Usage:
Let's say you have following models:

from django.db import models

class CarCompany(models.Model):
    name = models.CharField(max_length=128)

class CarModel(models.Model):
    name = models.CharField(max_length=64)
    company = models.ForeignKey(CarCompany, on_delete=models.CASCADE)

And you would like to filter results in CarModelAdmin on the basis of company. You need to define search_fields in CarCompany and then define filter like this:

from django.contrib import admin
from admin_searchable_dropdown.filters import AutocompleteFilter


class CarCompanyFilter(AutocompleteFilter):
    title = 'Company' # display title
    field_name = 'company' # name of the foreign key field


class CarCompanyAdmin(admin.ModelAdmin):
    search_fields = ['name'] # this is required for django's autocomplete functionality
    # ...


class CarModelAdmin(admin.ModelAdmin):
    list_filter = [CarCompanyFilter]
    # ...

After following these steps you may see the filter as:

1. This is how the list filter is rendered in the form of a dropdown when the package is used
   
2. And the dropdown filter is also Searchable

Features Offered:

1. If you have large fields in Admin Filter, the sidebar gets widened, this package provides you the same list in a dropdown format, hence, no such hassle.
2. If you have more than, say 20, field items, list filter is now a long side-pane, just ruining the admin interface. The Dropdown filter fixes that.
3. The Dropdown is also "Searchable", with an input text field (which utilizes Django's own auto_complete functionailty), so as long as the Django version you are using is greater than 2.0, you should be fine.
4. You can customize other list_filters you may have, like change the Title above the dropdown, or a custom Search logic etc.
5. You can customize Widget Texts to display in the Dropdown Option to use something other than the default str(obj)