How can I tell whether I'm on an add or edit page in Django's Admin?

Question

For the following code:

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    author = models.ForeignKey(Author)
    title = models.CharField(max_length=100)

class BookInline(admin.TabularInline):
    model = Book
    extra = 1

class AuthorAdmin(admin.ModelAdmin):
    inlines = [
        BookInline,
    ]

If I add a new Author through the admin, it will show me one Book entry because of the extra=1. If I edit an existing Author who has one Book, the admin will show the existing book and a new field to add a new one.

Current behaviour with Edit an Author who has 1 book:

Author: Someone
Book: The book title
Book #2:

Wanted behaviour with Edit an Author who has 1 book:

Author: Someone
Book: The book title

Is it possible in the admin.pyfor the above code to check whether I'm in the add or edit page? My goal is to set extra=1 for add and extra=0 for edit.

Answer 1

Perhaps you could have a function that returned a conditional value based on the quantity of Books for the given inline?

Something like...

class BookInline(admin.TabularInline):
    model = Book
    extra = extra_count

    def extra_count(self):
        if self.model.objects.count > 0:
            return 1
        else
            return 0

This is sort of an odd behavior you're requesting. Just out of curiosity, why exactly do you not want it to show an extra entry row in edit mode?

Answer 2

Nevermind, I just used max_num=1 instead of extra=1, it solves my issue. Here's the reference.