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## Appendix B. Legacy Hibernate Criteria Queries

## Note

This appendix covers the legacy Hibernate `org.hibernate.Criteria` API, which should be considered deprecated. New development should focus on the JPA `javax.persistence.criteria.CriteriaQuery` API. Eventually, Hibernate-specific criteria features will be ported as extensions to the JPA `javax.persistence.criteria.CriteriaQuery`. For details on the JPA APIs, see ???.

This information is copied as-is from the older Hibernate documentation.

Hibernate features an intuitive, extensible criteria query API.

## B.1. Creating a `Criteria` instance

The interface `org.hibernate.Criteria` represents a query against a particular persistent class. The `Session` is a factory for `Criteria` instances.

    Criteria crit = sess.createCriteria(Cat.class);
    crit.setMaxResults(50);
    List cats = crit.list();

## B.2. Narrowing the result set

An individual query criterion is an instance of the interface `org.hibernate.criterion.Criterion`. The class `org.hibernate.criterion.Restrictions` defines factory methods for obtaining certain built-in `Criterion` types.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "Fritz%") )
        .add( Restrictions.between("weight", minWeight, maxWeight) )
        .list();

Restrictions can be grouped logically.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "Fritz%") )
        .add( Restrictions.or(
            Restrictions.eq( "age", new Integer(0) ),
            Restrictions.isNull("age")
        ) )
        .list();

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) )
        .add( Restrictions.disjunction()
            .add( Restrictions.isNull("age") )
            .add( Restrictions.eq("age", new Integer(0) ) )
            .add( Restrictions.eq("age", new Integer(1) ) )
            .add( Restrictions.eq("age", new Integer(2) ) )
        ) )
        .list();

There are a range of built-in criterion types (`Restrictions` subclasses). One of the most useful allows you to specify SQL directly.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.sqlRestriction("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) )
        .list();

The `{alias}` placeholder will be replaced by the row alias of the queried entity.

You can also obtain a criterion from a `Property` instance. You can create a `Property` by calling `Property.forName()`:

    Property age = Property.forName("age");
    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.disjunction()
            .add( age.isNull() )
            .add( age.eq( new Integer(0) ) )
            .add( age.eq( new Integer(1) ) )
            .add( age.eq( new Integer(2) ) )
        ) )
        .add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk" } ) )
        .list();

## B.3. Ordering the results

You can order the results using `org.hibernate.criterion.Order`.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "F%")
        .addOrder( Order.asc("name") )
        .addOrder( Order.desc("age") )
        .setMaxResults(50)
        .list();

    List cats = sess.createCriteria(Cat.class)
        .add( Property.forName("name").like("F%") )
        .addOrder( Property.forName("name").asc() )
        .addOrder( Property.forName("age").desc() )
        .setMaxResults(50)
        .list();

## B.4. Associations

By navigating associations using `createCriteria()` you can specify constraints upon related entities:

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "F%") )
        .createCriteria("kittens")
            .add( Restrictions.like("name", "F%") )
        .list();

The second `createCriteria()` returns a new instance of `Criteria` that refers to the elements of the `kittens` collection.

There is also an alternate form that is useful in certain circumstances:

    List cats = sess.createCriteria(Cat.class)
        .createAlias("kittens", "kt")
        .createAlias("mate", "mt")
        .add( Restrictions.eqProperty("kt.name", "mt.name") )
        .list();

(`createAlias()` does not create a new instance of `Criteria`.)

The kittens collections held by the `Cat` instances returned by the previous two queries are not pre-filtered by the criteria. If you want to retrieve just the kittens that match the criteria, you must use a `ResultTransformer`.

    List cats = sess.createCriteria(Cat.class)
        .createCriteria("kittens", "kt")
            .add( Restrictions.eq("name", "F%") )
        .setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP)
        .list();
    Iterator iter = cats.iterator();
    while ( iter.hasNext() ) {
        Map map = (Map) iter.next();
        Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS);
        Cat kitten = (Cat) map.get("kt");
    }

Additionally you may manipulate the result set using a left outer join:

    		List cats = session.createCriteria( Cat.class )
                           .createAlias("mate", "mt", Criteria.LEFT_JOIN, Restrictions.like("mt.name", "good%") )
                           .addOrder(Order.asc("mt.age"))
                           .list();

This will return all of the `Cat`s with a mate whose name starts with "good" ordered by their mate's age, and all cats who do not have a mate. This is useful when there is a need to order or limit in the database prior to returning complex/large result sets, and removes many instances where multiple queries would have to be performed and the results unioned by java in memory.

Without this feature, first all of the cats without a mate would need to be loaded in one query.

A second query would need to retreive the cats with mates who's name started with "good" sorted by the mates age.

Thirdly, in memory; the lists would need to be joined manually.

## B.5. Dynamic association fetching

You can specify association fetching semantics at runtime using `setFetchMode()`.

    List cats = sess.createCriteria(Cat.class)
        .add( Restrictions.like("name", "Fritz%") )
        .setFetchMode("mate", FetchMode.EAGER)
        .setFetchMode("kittens", FetchMode.EAGER)
        .list();

This query will fetch both `mate` and `kittens` by outer join. See ??? for more information.

## B.6. Components

To add a restriction against a property of an embedded component, the component property name should be prepended to the property name when creating the `Restriction`. The criteria object should be created on the owning entity, and cannot be created on the component itself. For example, suppose the `Cat` has a component property `fullName` with sub-properties `firstName` and `lastName`:

    		List cats = session.createCriteria(Cat.class)
    			.add(Restrictions.eq("fullName.lastName", "Cattington"))
    			.list();

Note: this does not apply when querying collections of components, for that see below Section B.7, “Collections”

## B.7. Collections

When using criteria against collections, there are two distinct cases. One is if the collection contains entities (eg. `<one-to-many/>` or `<many-to-many/>`) or components (`<composite-element/>` ), and the second is if the collection contains scalar values (`<element/>`). In the first case, the syntax is as given above in the section Section B.4, “Associations” where we restrict the `kittens` collection. Essentially we create a `Criteria` object against the collection property and restrict the entity or component properties using that instance.

For queryng a collection of basic values, we still create the `Criteria` object against the collection, but to reference the value, we use the special property "elements". For an indexed collection, we can also reference the index property using the special property "indices".

    		List cats = session.createCriteria(Cat.class)
    			.createCriteria("nickNames")
    				.add(Restrictions.eq("elements", "BadBoy"))
    			.list();

## B.8. Example queries

The class `org.hibernate.criterion.Example` allows you to construct a query criterion from a given instance.

    Cat cat = new Cat();
    cat.setSex('F');
    cat.setColor(Color.BLACK);
    List results = session.createCriteria(Cat.class)
        .add( Example.create(cat) )
        .list();

Version properties, identifiers and associations are ignored. By default, null valued properties are excluded.

You can adjust how the `Example` is applied.

    Example example = Example.create(cat)
        .excludeZeroes()           //exclude zero valued properties
        .excludeProperty("color")  //exclude the property named "color"
        .ignoreCase()              //perform case insensitive string comparisons
        .enableLike();             //use like for string comparisons
    List results = session.createCriteria(Cat.class)
        .add(example)
        .list();

You can even use examples to place criteria upon associated objects.

    List results = session.createCriteria(Cat.class)
        .add( Example.create(cat) )
        .createCriteria("mate")
            .add( Example.create( cat.getMate() ) )
        .list();

## B.9. Projections, aggregation and grouping

The class `org.hibernate.criterion.Projections` is a factory for `Projection` instances. You can apply a projection to a query by calling `setProjection()`.

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.rowCount() )
        .add( Restrictions.eq("color", Color.BLACK) )
        .list();

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.projectionList()
            .add( Projections.rowCount() )
            .add( Projections.avg("weight") )
            .add( Projections.max("weight") )
            .add( Projections.groupProperty("color") )
        )
        .list();

There is no explicit "group by" necessary in a criteria query. Certain projection types are defined to be grouping projections, which also appear in the SQL `group by` clause.

An alias can be assigned to a projection so that the projected value can be referred to in restrictions or orderings. Here are two different ways to do this:

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.alias( Projections.groupProperty("color"), "colr" ) )
        .addOrder( Order.asc("colr") )
        .list();

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.groupProperty("color").as("colr") )
        .addOrder( Order.asc("colr") )
        .list();

The `alias()` and `as()` methods simply wrap a projection instance in another, aliased, instance of `Projection`. As a shortcut, you can assign an alias when you add the projection to a projection list:

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.projectionList()
            .add( Projections.rowCount(), "catCountByColor" )
            .add( Projections.avg("weight"), "avgWeight" )
            .add( Projections.max("weight"), "maxWeight" )
            .add( Projections.groupProperty("color"), "color" )
        )
        .addOrder( Order.desc("catCountByColor") )
        .addOrder( Order.desc("avgWeight") )
        .list();

    List results = session.createCriteria(Domestic.class, "cat")
        .createAlias("kittens", "kit")
        .setProjection( Projections.projectionList()
            .add( Projections.property("cat.name"), "catName" )
            .add( Projections.property("kit.name"), "kitName" )
        )
        .addOrder( Order.asc("catName") )
        .addOrder( Order.asc("kitName") )
        .list();

You can also use `Property.forName()` to express projections:

    List results = session.createCriteria(Cat.class)
        .setProjection( Property.forName("name") )
        .add( Property.forName("color").eq(Color.BLACK) )
        .list();

    List results = session.createCriteria(Cat.class)
        .setProjection( Projections.projectionList()
            .add( Projections.rowCount().as("catCountByColor") )
            .add( Property.forName("weight").avg().as("avgWeight") )
            .add( Property.forName("weight").max().as("maxWeight") )
            .add( Property.forName("color").group().as("color" )
        )
        .addOrder( Order.desc("catCountByColor") )
        .addOrder( Order.desc("avgWeight") )
        .list();

## B.10. Detached queries and subqueries

The `DetachedCriteria` class allows you to create a query outside the scope of a session and then execute it using an arbitrary `Session`.

    DetachedCriteria query = DetachedCriteria.forClass(Cat.class)
        .add( Property.forName("sex").eq('F') );

    Session session = ....;
    Transaction txn = session.beginTransaction();
    List results = query.getExecutableCriteria(session).setMaxResults(100).list();
    txn.commit();
    session.close();

A `DetachedCriteria` can also be used to express a subquery. Criterion instances involving subqueries can be obtained via `Subqueries` or `Property`.

    DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class)
        .setProjection( Property.forName("weight").avg() );
    session.createCriteria(Cat.class)
        .add( Property.forName("weight").gt(avgWeight) )
        .list();

    DetachedCriteria weights = DetachedCriteria.forClass(Cat.class)
        .setProjection( Property.forName("weight") );
    session.createCriteria(Cat.class)
        .add( Subqueries.geAll("weight", weights) )
        .list();

Correlated subqueries are also possible:

    DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class, "cat2")
        .setProjection( Property.forName("weight").avg() )
        .add( Property.forName("cat2.sex").eqProperty("cat.sex") );
    session.createCriteria(Cat.class, "cat")
        .add( Property.forName("weight").gt(avgWeightForSex) )
        .list();

Example of multi-column restriction based on a subquery:

    DetachedCriteria sizeQuery = DetachedCriteria.forClass( Man.class )
        .setProjection( Projections.projectionList().add( Projections.property( "weight" ) )
                                                    .add( Projections.property( "height" ) ) )
        .add( Restrictions.eq( "name", "John" ) );
    session.createCriteria( Woman.class )
        .add( Subqueries.propertiesEq( new String[] { "weight", "height" }, sizeQuery ) )
        .list();

## B.11. Queries by natural identifier

For most queries, including criteria queries, the query cache is not efficient because query cache invalidation occurs too frequently. However, there is a special kind of query where you can optimize the cache invalidation algorithm: lookups by a constant natural key. In some applications, this kind of query occurs frequently. The criteria API provides special provision for this use case.

First, map the natural key of your entity using `<natural-id>` and enable use of the second-level cache.

    <class name="User">
        <cache usage="read-write"/>
        <id name="id">
            <generator class="increment"/>
        </id>
        <natural-id>
            <property name="name"/>
            <property name="org"/>
        </natural-id>
        <property name="password"/>
    </class>

This functionality is not intended for use with entities with mutable natural keys.

Once you have enabled the Hibernate query cache, the `Restrictions.naturalId()` allows you to make use of the more efficient cache algorithm.

    session.createCriteria(User.class)
        .add( Restrictions.naturalId()
            .set("name", "gavin")
            .set("org", "hb")
        ).setCacheable(true)
        .uniqueResult();

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