<%-- * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, * software distributed under the License is distributed on an * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY * KIND, either express or implied. See the License for the * specific language governing permissions and limitations * under the License. --%> <%@page import="openbook.server.OpenBookService"%> <%@page import="openbook.domain.Book"%> <%@page import="openbook.domain.ShoppingCart"%> <%@page import="openbook.domain.PurchaseOrder"%> <%@page import="openbook.domain.LineItem"%> <%@page import="java.util.Map"%> <%@page import="java.util.List"%> <%@page import="openbook.util.JSPUtility"%> <%@include file="header.jsp"%>

Composite Pattern and Derived Identity

You have just created a new Purchase Order . Each Book in the Shopping Cart is turned into separate line item for the order and the Purchase Order and all its line items are inserted as new database records. All this happened with this few lines of Java Code

Transitive Persistence: The line items are persisted without any explicit call to persist because persist operation cascades via the order-line item relation. During persist, the JPA provider generated a new identity of the Purchase Order automatically.
Compound, Derived identity: The identity generation, in this case, is more interesting if you look at the identity used by the line items. Line Item uses compound, derived identity. It is compound because more than one field make up the identity. It is derived because one of the identity field borrows its value from the owning Purchase Order's identity. Because of such dependency, the persistent identity of a Line Item can only be assigned only after a new identity value for a Purchase Order gets generated during transaction commit.
Composite Relation: Purchase Order - Line Item relationship is by semantics, a composite relation. Simply stated, life time of a Line Item is completely controlled by the owning Purchase Order. It is noteworthy that the Line Item constructor is package protected to ensure that only Purchase Order can create them.
The Java language provides no support for composite relationship -- but it is a common pattern for persistent objects. The new JPA features of derived identity combined with orphan delete (another new feature) and cascaded persistent operations provides an application to reliably express a classic Master-Details pattern in their application.

<% OpenBookService service = (OpenBookService)session.getAttribute(KEY_SERVICE); ShoppingCart cart = (ShoppingCart)session.getAttribute(KEY_CART); PurchaseOrder order = service.placeOrder(cart); %> Purchase Order ID : <%= order.getId() %>
Placed on : <%= JSPUtility.format(order.getPlacedOn()) %>
<% int i = 0; List items = order.getItems(); for (LineItem item : items) { %> <% } %>

<%= order.getItems().size() %> items
Title	Quantity	Price
Continue Shopping
<%= item.getBook().getTitle() %>	<%= item.getQuantity() %>	<%= JSPUtility.format(item.getBook().getPrice() * item.getQuantity()) %>
Total		<%= JSPUtility.format(order.getTotal()) %>

<%@include file="footer.jsp"%>