JPA, one-to-many list is always null

[Tom Gibbins]

Hello everyone, i am trying to make a simple database using JPA, and i just can't figure out what i'm doing wrong. I have two entites, Account and Person, where one Person can have many accounts, and one account can only have one person. My problem is that the list i use in Person to store the account is never initialized. I know it is initialized when fed to the database, and when i query the database i get data returned as expected, except the ArrayList<Account> account, which is always null. Can anyone please help me out?

Person entity:

@Entity(name = "Person")
@Table(name = "Person")
public class Person implements Serializable {
	
	@OneToMany(mappedBy="owner")
//this list is the problem, it is always null when i query a person (everything else is fine)
		private List<Account> account = new ArrayList<>();

	@Id
	@GeneratedValue
	private int id;
	@Column (name = "PersonId", length = 11)
	private String PersonId;
	@Column(name = "Name", length = 30)
	private String Name;
	@Column(name = "Address", length = 30)
	private String Address;
	@Column(name = "AreaCode", length = 4)
	private String AreaCode;
	@Column(name = "Area", length = 35)
	private String Area;

	/*** Constructors*/
	public Person () {}

	public Person(String personId, String name, String address, String areaCode, String area) {

		setPersonId(personId);
		setName(name);
		setAddress(address);
		setAreaCode(areaCode);
		setArea(area);
		}

//getters and setters omitted...

Account entity:

@Entity(name= "Account")
@Table(name = "Account")
public class Account implements Serializable {

	@Id
	@GeneratedValue
	private int id;
	@Column(name = "Name", length = 30)
	private String Name;
	@Column(name = "Balance",length = 20)
	private String Balance;
	@Column(name = "Date")
	private Date dateCreated;
	
	@ManyToOne(fetch=FetchType.EAGER)
	@JoinColumn(name="OWNER_ID")
	private Person owner;
	
	public Account() {}

	public Account(Person p, String name, String balance, Date date) {
		
		setOwner(p);
		setName(name);
		setBalance(balance);
		setDateCreated(date);
	}
//getters and setters omitted

EntityManager:

/**
	 * Add a new person to the database
	 */
	@TransactionAttribute(TransactionAttributeType.REQUIRED)
	public void add(Person person) throws Exception {
//list is initialized here, and prints - List: [] - list count: 0
		System.out.println(" - List: " + person.getAccount() + " - list count: " + person.getAccount().size());
		entityManager.persist(person);
		}

I can add more code if necessary, but hopefully it is enough. If i now add a person:

Person p = new Person("12345678910", "Alan Wright", "Goodback 12", "", "1111", "Somewhere");

And query this person, i get all relevant data - except the accounts list, which is null. Any idea why?

[Bear Bibeault]

Tom Gibbins wrote:And query this person...

As the query is the problem, perhaps you should show us this code. And also, the schema for the tables.

I assume there is data in the tables that relate multiple accounts to the person?

And I'm bit confused by your explanation. Generally, the object is created by the query. Why are you creating a new one by hand before the query?

[Bear Bibeault]

Nit: a list of accounts should probably be named accounts, rather than account.

[Tom Gibbins]

As the query is the problem, perhaps you should show us this code. And also, the schema for the tables.

A query is done like this:

/**
	 * Method for querying a person based on his/hers personId
	 */
	@Override
	@TransactionAttribute(TransactionAttributeType.REQUIRED)
	public Person find(String personId) {
		//todo: protect against SQL injection
		Person p = null;
		
		try {
			String pnumber = personId;
			
			Query query = entityManager.createQuery("SELECT p from Person as p WHERE p.PersonId LIKE :pnumber")
					.setParameter("pnumber ", pnumber);
			p = (Person) query.getSingleResult();
			return p;
		} catch(NoResultException ex) {
			System.out.println("Could not find any results with this pId");
		} catch(Exception e) {
			System.out.println("Error, something went wrong");
			e.printStackTrace();
		}
		return p;
	}

As for the schema, im not sure how to post it, isen't it inferred from my initial post?

I assume there is data in the tables that relate multiple accounts to the person?

Not in the example i posted, but in the real db there is, yes. But i don't see how that matter? Shouldn't the list be initialized but empty if there is no account?

[Bear Bibeault]

Does it work properly when there is data to populate the list?

[Tom Gibbins]

Bear Bibeault wrote:Does it work properly when there is data to populate the list?

No, the list is always null

[Bear Bibeault]

Tom Gibbins wrote:As for the schema, im not sure how to post it, isen't it inferred from my initial post?

Not really. That's your mapping of the schema. Given that there's a problem, it's possible that there's something awry with the mapping. Seeing the actual schema from the point of view of the DB can help us see if it's a mapping problem.

[Tom Gibbins]

Bear Bibeault wrote:

Tom Gibbins wrote:As for the schema, im not sure how to post it, isen't it inferred from my initial post?

Not really. That's your mapping of the schema. Given that there's a problem, it's possible that there's something awry with the mapping. Seeing the actual schema from the point of view of the DB can help us see if it's a mapping problem.

Okay, fair point. But i'm not sure how to view that..

[Tom Gibbins]

Oh by the way, the persistence.xml is like this:

<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">
	<persistence-unit name="bankdb-unit" transaction-type="TRANSACTION">
		<jta-data-source>BankDBDatasource</jta-data-source>
		<non-jta-data-source>Unmanaged_BankDBDataSource</non-jta-data-source>
		<class>entity.Person</class>
		<class>entity.Account</class>
		<properties>
			<property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)" />
		</properties>
	</persistence-unit>
</persistence>

I don't know much about how this file works really, could the problem be here?

[Bear Bibeault]

Tom Gibbins wrote:But i'm not sure how to view that..

If you are working with databases you should have a DB manipulating tool on hand. Some good paid ones are DbVisualizer or DataGrip. Squirrel SQL is still be available and is free (donate-ware), albeit a bit more primitive. I'm sure there are many more to choose from.

These tools are essential to not only viewing database schemas, but can help you manipulate the data and the schemas.

[Tom Gibbins]

I figured it out, if i change

 private List<Account> account = new ArrayList<>();

to

 private Collection<Account> account = new ArrayList<>();

it works. Not sure i understand why, but "List" was the problem

[Bear Bibeault]

That's not a fix, that's a kludge.

After you get the collection back, what is its type? (use the getClass() method)

[Tom Gibbins]

Bear Bibeault wrote:That's not a fix, that's a kludge.

After you get the collection back, what is its type? (use the getClass() method)

Collection class: org.apache.openjpa.util.java$util$ArrayList$proxy

[Bear Bibeault]

Looks like it's an ArrayList implementation and should be assignable to List.