Nsryjdtyk

Data Model:

• Each Library has many Books


• Each Book may or may not be read by a User dictated by the UserBookReceipt
  

Objective:

Given a user, order the libraries by how many books they have read at each library.

Example:

User 1 has read:

• 2 Books from Library 1


• 5 Books from Library 2


• 1 Book from Library 3


• 0 Books from Library 4

We should return the libraries in order of number of books read by User 1 ie:

Library 2, Library 1, Library 3

Current solution

libraries = (UserBookReceipt.objects

    .filter(user=user)

    .values('book__library')

    .annotate(rcount=Count('book__library'))

    .order_by('-rcount')

)

Actual output

[

    {'book_library': 2, 'rcount': 5}, 

    {'book_library': 1, 'rcount': 2}, 

    {'book_library': 3, 'rcount': 1}

]

Expected output

[

    {'book_library': <Library: 2>, 'rcount': 5}, 

    {'book_library': <Library: 1>, 'rcount': 2}, 

    {'book_library': <Library: 3>, 'rcount': 1}

]

The actual output is 90% of what I want, except I want the book_library value to be instances of the django Library model rather than just the library id. Otherwise, I have to make another query to retrieve the Library objects which would probably require some inefficient ids__in query.

How can I count and group by the UserBookReceipt.book__library and return the Library model?

If I were to do this in SQL the query would look like

with rcount_per_lib as (

    select lib.id, count(*) as rcount

    from lib, books, user_book_receipts

    where user_book_receipts.book_id = books.id

        and lib.id = books.lib_id 

        and user_book_receipts.user_id = 1

    group by lib.id)

select lib.*, rcount from rcount_per_lib 

where lib.id = rcount_per_lib.id 

order by rcount

You need to change how you approach the queryset. Use Library rather than UserBookReceipt.

libraries = (Library.objects

    .filter(book__userbookreceipt__user=user)

    .annotate(rcount=Count('book__userbookreceipt', distinct=True))

    .order_by('-rcount')

)

[x.rcount for x in libraries]