from django.db.models import Q, F
from django.db import models

from apps.books.models import Book
from apps.shelves.models import Shelf

class ShelfBook(models.Model):
    book = models.ForeignKey(Book, on_delete=models.CASCADE, related_name="shelf_items")
    shelf = models.ForeignKey(Shelf, on_delete=models.CASCADE, related_name="book_items")

    quantity = models.PositiveIntegerField(default=0)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=["book", "shelf"],
                name="unique_book_per_shelf"
            ),
        ]