Autocomplete and populating Subcategories based on selected Categories (Admin and Forms)

[Jon Levischi]

A combination of the recently implemented "autocomplete_fields" with dynamic "limit_choices_to" that actually works on Foreign AND ManyToMany fields both on Admin site and Custom Forms.

class Project
    categories = M2M [having "Cat A", "Cat B", "Cat C", "Cat D", etc.]
    subcategories = M2M [having "Sub A1", "Sub A2", "Sub B1", "Sub B2", "Sub C1", "Sub C2", etc.]

When selecting Cat A in the category it would populate subcategory limiting options to "Sub A1", "Sub A2", "Sub C1", "Sub C2", based on their parents, but not "Cat B" nor "Cat D" children).

I know this is possible using Ajax, django-autocomplete-light or similar, but it's so frequently used that I think it makes sense to be included in Django itself.

Thank you!