Add position for book in collection

internal/routes/collectionaddbookpost.go

@@ -56,7 +56,7 @@ func PostCollectionBookHandler(ac appcontext.AppContext) {
 		return
 	}
 
-	collection.Books = append(collection.Books, book)
-	ac.Db.Save(&collection)
+	item := model.CollectionItem{Position: 0, BookID: book.ID, CollectionID: collection.ID}
+	ac.Db.Save(&item)
 	ac.C.String(http.StatusOK, "Success")
 }