Hibernate count collection size without initializing

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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() and contains().
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(), or isEmpty() — the database is queried to retrieve the necessary information. If it’s a Map or a List, the operations containsKey()
and get() 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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