如何在 Hibernate 中选择没有子对象的父对象？

Question

I have the following entities, parent and child, it's possible that a parent object doesn't have children. How to select parents that don't have children in hibernate? It would be similar to the not existing clause in SQL. Here are my entities:

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

    @Id
    @Column(name="PARENT_ID")
    private Long parentId;

    @Column(name="NAME")
    private String name;

    @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
    Set<Child> children;
}

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

    @Id
    @Column(name="CHILD_ID")
    private Long childId;

    @Column(name="NAME")
    private String name;

    @ManyToOne(cascade={CascadeType.ALL}, fetch=FetchType.EAGER)
    @JoinColumn(name="PARENT_ID", nullable=false, updatable=false)
    private Parent parent;
}

My query:

public List<Parent> getParentsNotHavingChildren() {
    Session session = this.sessionFactory.getCurrentSession();
    Transaction txn = session.beginTransaction();
    List<Parent> result = null;
    Criteria criteria = session.createCriteria(Parent.class)
            .add(Restrictions.eq("children", null));
    result = criteria.list();
    txn.commit();
    if (session.isOpen()) {
        session.close();
    }
    log.info(String.format("Get %s parents", result.size()));
    return result;
}

Then I got errors

org.hibernate.exception.GenericJDBCException: could not execute query
    at org.hibernate.exception.SQLStateConverter.handledNonSpecificException(SQLStateConverter.java:140)
    at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:128)
    at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
    at org.hibernate.loader.Loader.doList(Loader.java:2545)
    at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2276)
    at org.hibernate.loader.Loader.list(Loader.java:2271)
    at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:119)
    at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1716)
    at org.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:347)
    at HibernateParentDao.getParentsNotHavingChildren(HibernateParentDao.java:42)
Caused by: java.sql.SQLException: Missing IN or OUT parameter at index:: 1
    at oracle.jdbc.driver.SQLStateMapping.newSQLException(SQLStateMapping.java:70)
    at oracle.jdbc.driver.DatabaseError.newSQLException(DatabaseError.java:110)
    at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:171)
    at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:227)
    at oracle.jdbc.driver.OraclePreparedStatement.processCompletedBindRow(OraclePreparedStatement.java:1737)
    at oracle.jdbc.driver.OraclePreparedStatement.executeInternal(OraclePreparedStatement.java:3376)
    at oracle.jdbc.driver.OraclePreparedStatement.executeQuery(OraclePreparedStatement.java:3425)
    at oracle.jdbc.driver.OraclePreparedStatementWrapper.executeQuery(OraclePreparedStatementWrapper.java:1202)
    at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeQuery(NewProxyPreparedStatement.java:76)
    at org.hibernate.jdbc.AbstractBatcher.getResultSet(AbstractBatcher.java:208)
    at org.hibernate.loader.Loader.getResultSet(Loader.java:1953)
    at org.hibernate.loader.Loader.doQuery(Loader.java:802)
    at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:274)
    at org.hibernate.loader.Loader.doList(Loader.java:2542)

I'm sure my criteria isn't correct, but I can't find what should be the right way of this type of query. Any help would be very appreciated!

Answer 1

To test an attribute to be null, then use isNull ... change

Criteria criteria = session.createCriteria(Parent.class)
        .add(Restrictions.eq("children", null));

for

Criteria criteria = session.createCriteria(Parent.class)
        .add(Restrictions.isNull("children"));

Otherwise, Hibernate will expect a non-null value to perform the eq

Answer 2

This is a simple criteria to select a parent with no children (for one to many relations you should use isEmpty ).

Criteria criteria = currentSession().createCriteria(Parent.class);
criteria.add(Restrictions.isEmpty("children"));
return criteria.list();

And by the way, for a simple "select" query you don't need to create a transaction . Just with the three lines above it should be enough.

The other answer may work but only in one to one relationships. Hope it works!