Cleaned up and refactored ModelAdmin.formfield_for_dbfield:

* The new method uses an admin configuration option (`formfield_overrides`); this makes custom admin widgets especially easy.
  * Refactored what was left of `formfield_for_dbfield` into a handful of smaller methods so that it's easier to hook in and return custom fields where needed.
  * These `formfield_for_*` methods now pass around `request` so that you can easily modify fields based on request (as in #3987).

Fixes #8306, #3987, #9148.

Thanks to James Bennet for the original patch; Alex Gaynor and Brian Rosner also contributed.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9760 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/regressiontests/admin_widgets/widgetadmin.py

@@ -0,0 +1,22 @@
+"""
+
+"""
+from django.contrib import admin
+
+import models
+
+class WidgetAdmin(admin.AdminSite):
+    pass
+
+
+class CarTireAdmin(admin.ModelAdmin):
+    def formfield_for_foreignkey(self, db_field, request, **kwargs):
+        if db_field.name == "car":
+            kwargs["queryset"] = models.Car.objects.filter(owner=request.user)
+            return db_field.formfield(**kwargs)
+        return super(CarTireAdmin, self).formfield_for_foreignkey(db_field, request, **kwargs)
+
+site = WidgetAdmin()
+
+site.register(models.Car)
+site.register(models.CarTire, CarTireAdmin)