Added some logic

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
+
+
+