无需初始化的休眠计数集合大小 [英] Hibernate count collection size without initializing

问题描述

有没有一种方法可以在不初始化的情况下计算关联集合的大小?

Is there a way I can count the size of an associated collection without initializing?

例如

Select count(p.children) from Parent p

(我不能以任何其他方式执行此操作是有充分理由的，因为我的 where 子句更复杂，而我的 from 子句是多态查询)

(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)

谢谢.

推荐答案

除了查询之外的一个可能的解决方案可能是将 children 与 lazy="extra" 映射(在 XML 中)符号).通过这种方式，您可以使用您需要的任何查询获取 Parent，然后调用 parent.getChildren().size() 而不加载整个集合(只有 SELECT COUNT 类型查询被执行).

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).

如果有注释，那就是

@OneToMany
@org.hibernate.annotations.LazyCollection(
org.hibernate.annotations.LazyCollectionOption.EXTRA
)
private Set<Child> children = new HashSet<Child>();

更新:引自 Java Persistence with Hibernate，ch.13.1.3:

Update: Quote from Java Persistence with Hibernate, ch. 13.1.3:

如果调用任何不是标识符 getter 的方法，则代理将被初始化方法，如果您开始迭代其元素或如果您可以调用任何集合管理操作，例如 size() 和 contains().Hibernate 提供了一个额外的设置，对于大型集合最有用；它们可以被映射为额外懒惰.[...]

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. [...]

[映射如上，]如果调用 size()、contains() 或 isEmpty()，则集合不再初始化> — 查询数据库以检索必要的信息.如果是 Map 或 List，则操作 containsKey()而get()也直接查询数据库.

[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.

所以有了上面映射的实体，你就可以做

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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