为什么即使数据库中包含实体，"EntityManager.contains(..)"也返回false? [英] Why does 'EntityManager.contains(..)' return false even if an entity is contained in DB?

问题描述

我使用了此 JPA:检查实体对象是否已保留 知道我是坚持还是合并实体，它看起来像这样:

I used this JPA: check whether an entity object has been persisted or not to know if i persist or merge my entity , It will look like this :

if (!getEntityManager().contains(entity)) {
        System.out.println(" PERSIST ");            
    } else {
        System.out.println(" MERGE ");
    }

情况是-即使我编辑我的实体-也不会将其识别为合并.

The case is that - even if I edit my entity - it will not recognized as a merge.

这怎么可能以及如何使其起作用?

How is it possible and how to make it work?

推荐答案

根据

According to the JPA 2.1 specification (PDF page 72),

EntityManager方法public boolean contains(Object entity)可以做到:

检查实例是否为属于当前持久性上下文的托管实体实例.

Check if the instance is a managed entity instance belonging to the current persistence context.

由于这个原因，不是针对实际数据库进行检查，而是针对当前 持久性上下文 .

For this reason, the check is not conducted against the actual database, but against the current persistence context.

此外，在规格文档的PDF第86页上，我们发现:

Moreover, on PDF page 86 of the spec document we find:

contains方法返回true:

•如果实体已从数据库中检索或由getReference返回，并且已未被删除或分离.

• If the entity has been retrieved from the database or has been returned by getReference, and has not been removed or detached.

•如果实体实例是新的，并且已在该实体上调用了persist方法，或者已将persisted操作级联到该实例.

• If the entity instance is new, and the persist method has been called on the entity or the persist operation has been cascaded to it.

contains方法返回false:

•如果实例是分离的.

在执行代码片段的调用代码的那一刻，您很可能具有分离的实体状态.因此，对contains(..)的调用始终评估为false.

Most likely, you have a detached entity state in the moment the calling code of the code snippet is executed. Thus, the call for contains(..) always evaluates to false.

或者，您可以使用

• public <T> T find(Class<T> entityClass, Object primaryKey)(请参阅第66页)或
• public <T> T getReference(Class<T> entityClass, Object primaryKey)(请参阅第68页)

• public <T> T find(Class<T> entityClass, Object primaryKey) (see p. 66) or
• public <T> T getReference(Class<T> entityClass, Object primaryKey) (see p. 68)

检查基础数据库中是否以元组的形式存在.您选择上述哪种方法取决于代码/应用程序的上下文.

to check the presence as a tuple in the underlying database. Which one of the above methods you chose will depend on the context of your code/application.

希望有帮助.

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