标准休眠加入

Question

I need to do a criteria that return a list of "b" object. I have the following classes:

---
A
---
private BigDecimal id;
private String name;
private B b;

---
B
---
private BigDecimal id;
private String name;

I doing this:

Criteria criteria = getSession().createCriteria(A.class,"a").createCriteria("b");
return criteria.list();

I always get a List< A >, and i don't know how would be to get List< B >. Is it that possible?

Edit: Using Java SE 1.7

Answer 1

If you don't want a bidirectional relation between A and B, use an alias and then collect just b properties.

List<A> aList = getSession().createCriteria(A.class, "a")
        .createAlias("a.b", "b")
        .add(Restrictions.eq("a.name", "A name")
        .add(Restrictions.eq("b.name", "B name")
        .list();
List<B> bList = new ArrayList<>(aList.size());
for (A a : aList) {
    bList.add(a.getB());
}

Answer 2

You can navigate to associations using a nested Criteria like this:

List<B> result = session.createCriteria( A.class )
     .add( Restrictions.like("name","he%") )
     .createCriteria( "b" )
         .add( Restrictions.like("name", "he%") )
     .list();

My "second" answer is not for Criteria but rather the CriteriaBuilder which I wouldn't recommend for small use cases but for use cases where a query needs to be created dynamically with changing conditions coming from users or other applications.

final CriteriaBuilder queryBuilder = getCriteriaBuilder(); // Retrieve your CriteriaBuilder here. I inject mine over CDI for example...
final CriteriaQuery<B> query = queryBuilder.createQuery( B.class ); // Type the query to it's expected end result type.

final Root<A> queryRoot = query.from( A.class ); // Select the root of the query
final Join<Object, Object> BJoin= queryRoot.join("b", JoinType.LEFT ); // "b" is the field name to use for the mapping between the root table to the joined table. In this case a.b
// The above equals "left join b on b.id = a.b.id "

// Perform a select with the Class resulting from the select and what wants to be selected. 
// It is also possible to select only a field of a table but in our case we want the whole table of B to be selected.
final Selection<B> select = queryBuilder.construct( B.class, BJoin ); 
query.select(select); // add the select to the query

// We need to remember the ParameterExpression in order to fill the where condition.
// This acts as a typed(!) blank to later fill with the condition we want to match
final ParameterExpression<String> bName = queryBuilder.parameter(String.class);

// Define the where condition using the Path<T> you retrieve from Root or Join objects.
// This will make hibernate build the condition for the correct table like b.name
final Predicate bPredicate = queryBuilder.equal( bJoin.get("name"),bName );

query.where(bPredicate); // add the where expression to the query.
// The above equals something like "where b.name = ?"

// Compile the built query to a TypedQuery
// The EntitiyManager is also injected over CDI in my usual case.
final TypedQuery<B> builtQuery = javax.persistence.EntityManager.createQuery(query);
builtQuery.setParameter(bName,"test"); // Fill in the ? of the where condition.

final List<B> resultList = builtQuery.getResultList();

This seems very heavy and complex in the beginning but it is possible to use this very dynamically, since you can just extract multiple methods out of the snippet and enable adding multiple where conditions, order by, group by etc.

Answer 3

Finally I did this:

public List<B> findXXXX(A a) {

    // A
    DetachedCriteria aCriteria = DetachedCriteria.forClass(A.class, "a");
    aCriteria.add(Restrictions.eq("a.name", "some name"));
    aCriteria.setProjection(Projections.property("a.key"));

    // B
    Criteria bcriteria = getSession().createCriteria(B.class, "b");
    bcriteria.add(Property.forName("a.key").in(aCriteria));

    return bcriteria.list();
}

A subquery because will took few records in database. Thanks for helping!