Предположу, что в модели ChatRoom есть many-to-many поле с именем users:
from django.urls import resolve
class MessageInline(admin.TabularInline):
model = Message
def get_user(self, request):
resolved = resolve(request.path_info)
if resolved.args:
return self.parent_model.objects.get(pk=resolved.args[0])
return None
def formfield_for_foreignkey(self, db_field, request, **kwargs):
if db_field.name == "chat_room":
user = self.get_user(request)
if user:
kwargs["queryset"] = ChatRoom.objects.filter(users=user)
return super().formfield_for_foreignkey(db_field, request, **kwargs)