adrian li adrian li - 8 days ago 6
Python Question

Django Admin list_display add column [count records of related ForeignKey]

I am new to Python and Django.

I was trying to count the foreign key items/records of a model and display it as a column in the Admin list_display.

So I have a model "Author", as Foreign key to model "Book".

Here's the models.py:

class Author(models.Model):

name = models.CharField(
max_length=100,
)

class Meta:
ordering = ['-name ']
verbose_name = _('Author')
verbose_name_plural = _('Authors')

def __str__(self):
return self.author

@models.permalink
def get_absolute_url(self):
return reverse('author_detail', kwargs={'pk': self.pk})

class Book(models.Model):
author = models.ForeignKey(
'Author',
related_name='menu_items',
verbose_name=_('author'),
)

bookname= models.CharField(
max_length=100,
)

publisher= models.CharField(
max_length=100,
)
class Meta:
ordering = ['-bookname']
verbose_name = _('Book')
verbose_name_plural = _('Books')

def __str__(self):
return self.bookname

@models.permalink
def get_absolute_url(self):
return ('book_detail', [self.pk])


Here's the admin.py

from django.contrib import admin
from .models import Author, Book

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

@admin.register(Author)
class LibraryAdmin(admin.ModelAdmin):
def NumberOfBooks(self, obj):
#I guess sth is wrong here when trying to count the book_set
return obj.book_set.count()

NumberOfBooks.short_description = "Books Count"
list_display = ['bookname', 'publisher', 'NumberOfBooks']
inlines = [BookInline]


I want to count how many books each Author wrote, and display like this on the second level of the admin page Home>Authors>Author:

Author | Books Count
Someone1 NumberOfBooks
Someone2 NumberOfBooks
Someone3 NumberOfBooks
Someone4 NumberOfBooks


However I get the error:

'Author' object has no attribute 'book_set'


1. did I do the obj.book_set.count() wrong?
I thought
obj.book_set
would refer to "book" under the same "author"(obj), sounds like I misunderstood it but I am not sure how to achieve what I intended to.

2. Instead, if I wanted to add a field to 'Authors' which would auto-count number of "book" related to each Author, should this be a better approach and so how to do it?

Answer

There should be a list with all the available fields on the error page and you should find a field called menu_items (that's what you put into related_namein your Book model).

But that might be not the best option. You should be able to override get_queryset() method on your LibraryAdmin(admin.ModelAdmin) class like this:

...
from django.db.models import Count
...

@admin.register(Loan)
class LibraryAdmin(admin.ModelAdmin):

    def get_queryset(self, request):
        qs = super(LibraryAdmin, self).get_queryset(request)
        return qs.annotate(books_count=Count('menu_items'))

    def books_count(self, inst):
        return inst.books_count

    list_display = ['bookname', 'publisher', 'books_count']
    inlines = [BookInline]

This way counting will be done by your DB.

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