JPA/Hibernate Parent/Child relationship

Question

I am quite new to JPA/Hibernate (Java in general) so my question is as follows (note, I have searched far and wide and have not come across an answer to this):

I have two entities:

Parent and Child (naming changed).

Parent contains a list of Children and Children refers back to parent.

e.g.

@Entity
public class Parent {

@Id
@Basic
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "PARENT_ID", insertable = false, updatable = false)
private int id;        

    /* ..... */

    @OneToMany(cascade = { CascadeType.ALL }, fetch = FetchType.LAZY)
    @JoinColumn(name = "PARENT_ID", referencedColumnName = "PARENT_ID", nullable = true)
    private Set<child> children;

    /* ..... */

}

@Entity
public class Child {

@Id
@Basic
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "CHILD_ID", insertable = false, updatable = false)
private int id; 

    /* ..... */

    @ManyToOne(cascade = { CascadeType.REFRESH }, fetch = FetchType.EAGER, optional = false)
@JoinColumn(name = "PARENT_ID", referencedColumnName = "PARENT_ID")
private Parent parent;

    /* ..... */

}

I want to be able to do the following:

Retrieve a Parent entity which would contain a list of all its children (List), however, when listing Parent (getting List, it of course should omit the children from the results, therefore setting FetchType.LAZY.
Retrieve a Child entity which would contain an instance of the Parent entity.

Using the code above (or similar) results in two exceptions:

Retrieving Parent: A cycle is detected in the object graph. This will cause infinitely deep XML...
Retrieving Child: org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: xxxxxxxxxxx, no session or session was closed

When retrieving the Parent entity, I am using a named query (i.e. calling it specifically) @NamedQuery(name = "Parent.findByParentId", query = "SELECT p FROM Parent AS p LEFT JOIN FETCH p.children where p.id = :id")

Code to get Parent (i.e. service layer):

public Parent findByParentId(int parentId) {
    Query query = em.createNamedQuery("Parent.findByParentId");
    query.setParameter("id", parentId);

    return (Parent) query.getSingleResult();
}

Why am I getting a LazyInitializationException event though the List property on the Parent entity is set as Lazy (when retrieving the Child entity)?

I prefer not using query.getSingleResult(), rather returning a list and checking for zero or one members. — ThomasRS
other theme: use return em.find(Parent.class, Integer.valueOf(parentId)); instead of using the query. — Ralph
other theme: use nullable ids like Integer, instead of primitive int. — Ralph
I hope it is a typo in the question, not in your code. But your parent child set refers to an other child than the entity class! -- The Entity is Child (upper case 'c') and the Set is of type child with lower case letter! — Ralph
@Ralph That is a typo! Should be private Set<Child> children — NubieJ

ThomasRS ThomasRS · Accepted Answer · 2011-03-17T15:08:43

Exception 1. Your XML serialization does not have anything with JPA/Hibernate, you will be able to serialize your two-way-setup successfully. If automatic serialization fails, preprocess your to-be serialized objects in such a way that you remove the link from child to parent - outside the transaction (assuming you have automatic merges in your container).

Exception 2. You should read up on Transactions. In short, there are rules to when traversal of objects requiring database interaction can be done and not. Logically this divide must exist. If you want lazy relationships to behave as 'normal' (read: fetched) outside the transaction session, you simple 'prefetch' the relationship by traversing or accessing it in such a way that the relationship is resolved. For example call .size, or iterating over the members, on a set or a list will do this.

JPA/Hibernate Parent/Child relationship

3 Answers