java - GAE 与 JPA : Update entity

Question

我在更新 Google App Engine 中的实体时遇到问题。

EntityManager em = ... // constructed like in the doc

MyEntity myE = new MyEntity();
myE.setType("1");      // String
em.persist(myE);em.refresh(myE);

myE.setType("2");
em.merge(myE);em.refresh(myE);

我期望一个具有 type="2" 的实体，但只有一个具有 type="1" 的实体:-(

Answer 1

这是正确的行为，让我解释一下(我假设您的所有代码都在相同的持久性上下文/事务中运行)。

# This line sets the value in the in-memory object without changing the database
myE.setType("2");

# this line doesn't do anything here, as the entity is already managed in the current 
# persistence context. The important thing to note is that merge() doesn't save the 
# entity to the DB.
em.merge(myE);

# This reloads the entity from the DB discarding all the in-memory changes.
em.refresh(myE);