7.3. Associations and joins
You may also assign aliases to associated entities, or even to elements of a collection of values, using a
select cat, mate, kitten from Cat as cat inner join cat.mate as mate left outer join cat.kittens as kitten
select cat from Cat as cat left join cat.mate.kittens as kittens
The supported join types are borrowed from ANSI SQL
left outer join
left outer joinconstructs may be abbreviated.
select cat, mate, kitten from Cat as cat join cat.mate as mate left join cat.kittens as kitten
In addition, a "fetch" join allows associations or collections of values to be initialized along with their parent objects, using a single select. This is particularly useful in the case of a collection. It effectively overrides the fetching options in the associations and collection mapping metadata. See the Performance chapter of the Hibernate reference guide for more information.
select cat from Cat as cat inner join fetch cat.mate left join fetch cat.kittens
A fetch join does not usually need to assign an alias, because the associated objects should not be used in the
whereclause (or any other clause). Also, the associated objects are not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason we might need an alias is if we are recursively join fetching a further collection:
select cat from Cat as cat inner join fetch cat.mate left join fetch cat.kittens child left join fetch child.kittens
Note that the
fetchconstruct may not be used in queries called using
iterate(). Nor should
fetchbe used together with
setFirstResult(). It is possible to create a cartesian product by join fetching more than one collection in a query (as in the example above), be careful the result of this product isn't bigger than you expect. Join fetching multiple collection roles also sometimes gives unexpected results for bag mappings, so be careful about how you formulate your queries in this case.
If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties immediately (in the first query) using
fetch all properties. This is Hibernate specific option:
select doc from Document doc fetch all properties order by doc.name
select doc from Document doc fetch all properties where lower(doc.name) like '%cats%'