14.3. Associations and joins
You can also assign aliases to associated entities or to elements of a collection of values using a
join
. For example:
from Cat as cat inner join cat.mate as mate left outer join cat.kittens as kitten
from Cat as cat left join cat.mate.kittens as kittens
from Formula form full join form.parameter param
The supported join types are borrowed from ANSI SQL:
inner join
left outer join
right outer join
full join
(not usually useful)
The
inner join
, left outer join
and right outer join
constructs may be abbreviated.
from Cat as cat join cat.mate as mate left join cat.kittens as kitten
You may supply extra join conditions using the HQL
with
keyword.
from Cat as cat left join cat.kittens as kitten with kitten.bodyWeight > 10.0
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 outer join and lazy declarations of the mapping file for associations and collections. See Section 19.1, “Fetching strategies” for more information.
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
where
clause (or any other clause). The associated objects are also not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason you might need an alias is if you are recursively join fetching a further collection:
from Cat as cat inner join fetch cat.mate left join fetch cat.kittens child left join fetch child.kittens
Important
The
fetch
construct cannot be used in queries called using iterate()
(though scroll()
can be used). Fetch
should not be used together with setMaxResults()
or setFirstResult()
, as these operations are based on the result rows which usually contain duplicates for eager collection fetching, hence, the number of rows is not what you would expect. Fetch
should also not be used together with impromptu with
condition. It is possible to create a cartesian product by join fetching more than one collection in a query, so take care in this case. Join fetching multiple collection roles can produce unexpected results for bag mappings, so user discretion is advised when formulating queries in this case. Finally, note that full join fetch
and right join fetch
are not meaningful.
If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties in the first query immediately using
fetch all properties
.
from Document fetch all properties order by name
from Document doc fetch all properties where lower(doc.name) like '%cats%'