Added some logic

backend/apps/shelf_items/models.py

@@ -1,3 +1,19 @@
+from django.db.models import Q, F
 from django.db import models
 
-# Create your models here.
+from apps.books.models import Book
+from apps.shelves.models import Shelf
+
+class ShelfItem(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"
+            ),
+        ]