Added some logic

backend/apps/shelf_items/apps.py

@@ -2,4 +2,4 @@ from django.apps import AppConfig
 
 
 class ShelfItemsConfig(AppConfig):
-    name = 'shelf_items'
+    name = 'apps.shelf_items'

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"
+            ),
+        ]

backend/apps/shelf_items/serializers.py

@@ -0,0 +1,77 @@
+from rest_framework import serializers
+
+from apps.shelf_items.models import ShelfItem
+from apps.books.models import Book
+from apps.shelves.models import Shelf
+from apps.shelves.serializers import ShelfSerializer
+from apps.books.serializers import BookSerializer
+
+class ShelfItemSerializer(serializers.ModelSerializer):
+    shelf = ShelfSerializer(read_only=True)
+    shelf_id = serializers.PrimaryKeyRelatedField(
+        queryset=Shelf.objects.all(),
+        source="shelf", 
+        write_only=True
+    )
+
+    book = BookSerializer(read_only=True)
+    book_id = serializers.PrimaryKeyRelatedField(
+        queryset=Book.objects.all(),
+        source="book", 
+        write_only=True
+    )
+
+    class Meta:
+        model = ShelfItem
+        fields = [
+            "id",
+            "shelf",
+            "shelf_id",
+            "book",
+            "book_id",
+            "quantity"
+        ]
+        read_only_fields = ["id"]
+
+    def __init__(self, instance=None, data=..., **kwargs):
+        super().__init__(instance, data, **kwargs)
+
+        request = self.context.get("request")        
+        if request and request.user:
+            self.fields["shelf_id"].queryset = Shelf.objects.filter(
+                user=request.user
+            )
+
+            self.fields["book_id"].queryset = Book.objects.filter(
+                user=request.user
+            )
+
+    def validate(self, attrs):
+        attrs = super().validate(attrs)
+
+        book = attrs.get("book")
+        shelf = attrs.get("shelf")
+
+        if book and shelf:
+            qs = ShelfItem.objects.filter(
+                book=book,
+                shelf=shelf
+            )
+            
+            if self.instance:
+                qs = qs.exclude(pk=self.instance.pk)
+
+            if qs.exists():
+                raise serializers.ValidationError(
+                    "This Book already exists in shelf!"
+                )
+                
+            if book.user_id != shelf.user_id:
+                raise serializers.ValidationError(
+                    "Book and Shelf must be from the same user!"
+                )
+        
+        return attrs
+
+
+    

backend/apps/shelf_items/urls.py

@@ -0,0 +1,9 @@
+from rest_framework.routers import DefaultRouter
+
+from apps.shelf_items.views import ShelfItemsViewSet
+
+urlpatterns = []
+
+router = DefaultRouter()
+router.register("", viewset=ShelfItemsViewSet)
+urlpatterns += router.urls

backend/apps/shelf_items/views.py

@@ -1,3 +1,20 @@
-from django.shortcuts import render
+from rest_framework import viewsets
 
-# Create your views here.
+from apps.shelf_items.models import ShelfItem
+from apps.shelf_items.serializers import ShelfItemSerializer
+from rest_framework import viewsets 
+from rest_framework.permissions import IsAuthenticated
+from rest_framework_simplejwt.authentication import JWTAuthentication
+
+
+class ShelfItemsViewSet(viewsets.ModelViewSet):
+    queryset = ShelfItem.objects.all()
+    serializer_class =  ShelfItemSerializer 
+
+    authentication_classes = [JWTAuthentication]
+    permission_classes = [IsAuthenticated]
+
+    def get_queryset(self):
+        return ShelfItem.objects.filter(
+            shelf__user_id=self.request.user.pk,
+        ).select_related("book", "shelf").prefetch_related("book__authors")