Is there a way I can count the size of an associated collection without initializing?
e.g.
Select count(p.children) from Parent p
(there is a good reason why I cant do this any other way as my where clause is more complicated and my from clause is a polymorphic query)
Thanks.
A possible solution other than queries might be mapping children
with lazy="extra"
(in XML notation). This way, you can fetch the Parent with whatever query you need, then call parent.getChildren().size()
without loading the whole collection (only a SELECT COUNT
type query is executed).
With annotations, it would be
@OneToMany @org.hibernate.annotations.LazyCollection( org.hibernate.annotations.LazyCollectionOption.EXTRA ) private Set<Child> children = new HashSet<Child>();
Update: Quote from Java Persistence with Hibernate, ch. 13.1.3:
A proxy is initialized if you call any method that is not the identifier getter method, a collection is initialized if you start iterating through its elements or if you call any of the collection-management operations, such as
size()
andcontains()
. Hibernate provides an additional setting that is mostly useful for large collections; they can be mapped as extra lazy. [...][Mapped as above,] the collection is no longer initialized if you call
size()
,contains()
, orisEmpty()
— the database is queried to retrieve the necessary information. If it’s aMap
or aList
, the operationscontainsKey()
andget()
also query the database directly.
So with an entity mapped as above, you can then do
Parent p = // execute query to load desired parent // due to lazy loading, at this point p.children is a proxy object int count = p.getChildren().size(); // the collection is not loaded, only its size
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