JPA get values from the collections in entities

Question

Hi,

I'm new to JPA and now stuck with a problem!

I have two tables Person and PersonAddress.

In the Person entity, I have

@OneToMany(mappedBy="personid")
private Set<Personaddress> personaddressCollection;

and

public Set<Personaddress> getPersonaddressCollection() {
    return this.personaddressCollection;
}

public void setPersonaddressCollection(Set<Personaddress> personaddressCollection) {
    this.personaddressCollection = personaddressCollection;
}

In the PersonAddress entity, I have

@ManyToOne
@JoinColumn(name="PERSONID")
private Person personid;

I'm doing a query similar to one below:

List<Person> personlist = em.createQuery("SELECT e FROM Person e").getResultList();

I am expecting it to return all the data from Person table along with the data in PersonAddress table in the Set available in Person entity. The size of personlist is correct, but when I try to read the PersonAddress collection, I'm getting null values. But the database has values in it and it cannot be null.

Corresponding to every Partner, there will be a PartnerAddress which will not be null. How do I query it with JPA if whatever I have done so far is wrong?

Please help.

Answer 1

I am expecting it to return all the data from Person table along with the data in PersonAddress table in the Set available in Person entity.

A OneToMany is LAZY by default so a SELECT e FROM Person e won't load the collection of associated addresses.

If you want to change this behavior, either make the association eager:

@OneToMany(mappedBy="personid", fetch=FetchType.EAGER)
private Set<Personaddress> personaddressCollection;

Or prefetch the association using a FETCH JOIN:

SELECT e FROM Person e LEFT JOIN FETCH e.personaddressCollection

With the later approach, you can still benefit from lazy loading when you don't need the addresses. The former approach changes the behavior "globally".

The size of personlist is correct, but when I try to read the PersonAddress collection, I'm getting null values. But the database has values in it and it cannot be null.

This is somehow "another" issue. My recommendation would be to activate logging of SQL statements to see what query is performed exactly and why you don't get the corresponding addresses. There must be something wrong with the data.

References

• JPA 1.0 Specification
  • Section 9.1.24 "OneToMany Annotation"
  • Section 4.4.5.3 "Fetch Joins"