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POJPA persist entities with one to many relation
 

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    <p>Config</p> <ul> <li>EcliplseLink 2.3.2</li> <li>JPA 2.0</li> <li>The entities are auto created from the db schema from netbeans with <em>Entity Classes from Database...</em> wizard.</li> <li>The controller classes are auto created from netbeans with <em>JPA Controller Classes from Entity Classes...</em> wizard</li> </ul> <p><strong>Short version of question</strong></p> <p>In a classic scenario, two tables with one to many relation. I create the parent entity, then the child entity and I attach the child to the parent's collection. When I <em>create</em> (controller method) the parent entity, I expect the child entity to be created to and associated with parent. Why doesn't it happen?</p> <p><strong>Long version</strong></p> <p>Parent class</p> <pre><code>@Entity @XmlRootElement public class Device implements Serializable { private static final long serialVersionUID = 1L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) @Basic(optional = false) private Integer id; @Column(unique=true) private String name; @Temporal(TemporalType.TIMESTAMP) private Date updated; @OneToMany(cascade = CascadeType.ALL, mappedBy = "deviceId") private Collection&lt;NetworkInterface&gt; networkInterfaceCollection; public Device() { } public Device(String name) { this.name = name; updated = new Date(); } // setters and getters... @XmlTransient public Collection&lt;NetworkInterface&gt; getNetworkInterfaceCollection() { return networkInterfaceCollection; } public void setNetworkInterfaceCollection(Collection&lt;NetworkInterface&gt; networkInterfaceCollection) { this.networkInterfaceCollection = networkInterfaceCollection; } public void addNetworkInterface(NetworkInterface net) { this.networkInterfaceCollection.add(net); } public void removeNetworkInterface(NetworkInterface net) { this.networkInterfaceCollection.remove(net); } // other methods } </code></pre> <p>Child class</p> <pre><code>@Entity @Table(name = "NETWORK_INTERFACE") @XmlRootElement public class NetworkInterface implements Serializable { private static final long serialVersionUID = 1L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) @Basic(optional = false) private Integer id; private String name; @Temporal(TemporalType.TIMESTAMP) private Date updated; @JoinColumn(name = "DEVICE_ID", referencedColumnName = "ID") @ManyToOne(optional = false) private Device deviceId; public NetworkInterface() { } public NetworkInterface(String name) { this.name = name; this.updated = new Date(); } // setter and getter methods... public Device getDeviceId() { return deviceId; } public void setDeviceId(Device deviceId) { this.deviceId = deviceId; } } </code></pre> <p>Main class</p> <pre><code>public class Main { public static void main(String[] args) { EntityManagerFactory emf = Persistence.createEntityManagerFactory("wifi-dbPU"); DeviceJpaController deviceController = new DeviceJpaController(emf); NetworkInterfaceJpaController netController = new NetworkInterfaceJpaController(emf); Device device = new Device("laptop"); NetworkInterface net = new NetworkInterface("eth0"); device.getNetworkInterfaceCollection().add(net); deviceController.create(device); } } </code></pre> <p>This class throws a NullPointerException in line: <code>device.getNetworkInterfaceCollection().add(net);</code></p> <p>The system knows that there is a new entity <code>device</code> and it has an element <code>net</code> in it's collection. I expected it to write <code>device</code> in db, get device's id, attach it to <code>net</code> and write it in db.</p> <p>Instead of this, I found that these are the steps I have to do:</p> <pre><code>deviceController.create(device); net.setDeviceId(device); device.getNetworkInterfaceCollection().add(net); netController.create(net); </code></pre> <p><strong>Why do I have to create the child when the parent class knows it's child and it should create it for me?</strong></p> <p>The create method from DeviceJpaController (sorry for the long names in fields, they are auto generated).</p> <pre><code>public EntityManager getEntityManager() { return emf.createEntityManager(); } public void create(Device device) { if (device.getNetworkInterfaceCollection() == null) { device.setNetworkInterfaceCollection(new ArrayList&lt;NetworkInterface&gt;()); } EntityManager em = null; try { em = getEntityManager(); em.getTransaction().begin(); Collection&lt;NetworkInterface&gt; attachedNetworkInterfaceCollection = new ArrayList&lt;NetworkInterface&gt;(); for (NetworkInterface networkInterfaceCollectionNetworkInterfaceToAttach : device.getNetworkInterfaceCollection()) { networkInterfaceCollectionNetworkInterfaceToAttach = em.getReference(networkInterfaceCollectionNetworkInterfaceToAttach.getClass(), networkInterfaceCollectionNetworkInterfaceToAttach.getId()); attachedNetworkInterfaceCollection.add(networkInterfaceCollectionNetworkInterfaceToAttach); } device.setNetworkInterfaceCollection(attachedNetworkInterfaceCollection); em.persist(device); for (NetworkInterface networkInterfaceCollectionNetworkInterface : device.getNetworkInterfaceCollection()) { Device oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface = networkInterfaceCollectionNetworkInterface.getDeviceId(); networkInterfaceCollectionNetworkInterface.setDeviceId(device); networkInterfaceCollectionNetworkInterface = em.merge(networkInterfaceCollectionNetworkInterface); if (oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface != null) { oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface.getNetworkInterfaceCollection().remove(networkInterfaceCollectionNetworkInterface); oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface = em.merge(oldDeviceIdOfNetworkInterfaceCollectionNetworkInterface); } } em.getTransaction().commit(); } finally { if (em != null) { em.close(); } } } </code></pre>
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Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

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