Document that ModelAdmin.prepopulated_fields only works on empty forms

[Ju]

ModelAdmin.prepopulated_fields auto generating works only while creating new records but not while editing them.

from django.db import models

class Item(models.Model):
    title = models.CharField(max_length=200, blank=False)
    slug = models.SlugField(max_length=200)

from django.contrib import admin
from .models import Item

class ItemAdmin(admin.ModelAdmin):
   
  prepopulated_fields = {"slug": ("title",)}

admin.site.register(Item, ItemAdmin)

[Tim Graham]

That's expected behavior -- prepopulated_fields only works on empty forms. I think the idea is that slugs for existing objects shouldn't change since that could cause a URL to change. This could be documented.

[Botond Béres]

Opened a ​small PR which adds in the prepopulated_fields docs that it only works on empty forms.

[Tim Graham <timograham@…>]

In 56b364ba:

Fixed #28205 -- Doc'd that ModelAdmin.prepopulated_fields only works on add forms.

[Tim Graham <timograham@…>]

In 5630f4e6:

[2.0.x] Fixed #28205 -- Doc'd that ModelAdmin.prepopulated_fields only works on add forms.

Backport of 56b364bacca0d6a6cd8fbcd93aeb49f3ad46e58c from master

[Tim Graham <timograham@…>]

In 682cdf6c:

Refs #28205 -- Corrected ModelAdmin.prepopulated_fields docs regarding when they're populated.

[Tim Graham <timograham@…>]

In a9d9680e:

[2.1.x] Refs #28205 -- Corrected ModelAdmin.prepopulated_fields docs regarding when they're populated.

Backport of 682cdf6cab8cb76ef1808df45631c39748052e13 from master.