java 没有主键的表的 JPA 实体
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JPA entity for a table without primary key
提问by tputkonen
I have a MySQL table without primary key, and I have to map it into a JPA entity. I cannot modify the table in any way.
我有一个没有主键的 MySQL 表，我必须将它映射到一个 JPA 实体中。我无法以任何方式修改表格。
Because entities must have a primary key, I have to specify one. If I'm certain that the field I use as a primary key in the entity (or the fields, should I opt for using composite primary key) will always be unique (and not null) in table, can the fact that the table doesn't have a primary key specified in CREATE TABLE cause any issues?
因为实体必须有一个主键，所以我必须指定一个。如果我确定我在实体中用作主键的字段（或字段，我是否应该选择使用复合主键）在表中将始终是唯一的（而不是 null），表中的事实是否可以在 CREATE TABLE 中没有指定主键会导致任何问题吗？
回答by skaffman
That's correct. JPA has no way of knowing if the column(s) it is using as a PK is actually a real PK in the database. If those column(s) are, in practice, a PK, then it should be fine.
没错。JPA 无法知道它用作 PK 的列是否实际上是数据库中的真实 PK。如果这些列实际上是 PK，那么应该没问题。
You may potentially get some performance problems if the pseudo-PK columns are not correctly indexed, though - JPA will execute queries against the PK on the assumption that it will perform well.
但是，如果伪 PK 列没有正确索引，您可能会遇到一些性能问题 - JPA 将在假设它会执行良好的情况下对 PK 执行查询。
回答by Dewfy
JPA itself doesn't analyze your database. Just don't use common methods using primary key (find/merge/...) instead use named queries, for example using jpql update syntax.
JPA 本身不会分析您的数据库。只是不要使用使用主键 (find/merge/...) 的常用方法，而是使用命名查询，例如使用 jpql 更新语法。
@Entity
@Table(name = "login")
@NamedQueries({
@NamedQuery(name = "Login.updateLastOnline",
query = "UPDATE Login l SET l.lastOnline = :newDate WHERE l.loginId = :loginId")
})
public class Login implements Serializable
{
It doesn't matter if loginId is primary key
loginId 是否为主键无关紧要
