Question

I want to combine an object and a QuerySet in Django.

My objects are as follows:

class Book(models.Model):
    title = models.CharField(max_length=400)
    library_id = models.FloatField(max_length=200, null=True, blank=True, unique=True)
    copy_id = models.FloatField(max_length=200, null=True, blank=True)

My query is as follows (I’m looking up a book by ID, then finding all books that are copies, i.e. share a copy_id with the library_id of this book:

    primary_book = Book.objects.get(id=book_id)
    book_copies = Book.objects.filter(copy_id=primary_book.library_id)
    if book_copies:
        books = primary_book | book_copies
    else:
        books = primary_book

Perhaps there’s a better way to run the query? But at the moment I’m a bit stuck, because this is producing TypeError: unsupported operand type(s) for |: 'Book' and 'QuerySet'.

Thanks.

Answer 1

[Edit] If you don’t require a queryset:

books = [primary_book] + list(book_copies)

You could try using this if you do need a qs:

primary_books = Book.objects.filter(id=book_id)
book_copies = Book.objects.filter(copy_id=primary_books[0].library_id)
if book_copies:
    books = primary_books | book_copies
else:
    books = primary_books[0]

Or if you wanted to redo your model since it looks like your copy_id to library_id is a many to one relationship:

models.py

class Book(models.Model):
    library_id = models.FloatField(max_length=200, null=True, blank=True, unique=True)
    title = models.CharField(max_length=400)
    primary_book = models.ForeignKeyField('self', to_field='library_id', related_name='book_copies', null=True, blank=True)

views.py

from django.db.models import Q
books = Book.objects.filter(Q(id=book_id)|Q(primary_book__id=book_id))