Question

实体（使用Set）：

@Entity
class Product(
    @Id
    @GeneratedValue
    val int: Int = 0,
    val name : String,
    @ManyToMany(cascade = [(CascadeType.PERSIST), (CascadeType.MERGE)])
    val stores:  MutableSet<Store> = mutableSetOf()
)

@Entity
class Store(
        @Id
        @GeneratedValue
        val int: Int = 0,
        val name : String = ""
)

...

val p = Product(name = "product")
        em.persist(p)
        val store = Store(name = "store");
        p.stores += store
        em.persist(store)
        for (i in 1..5) {
            val s = Store(name = i.toString())
            p.stores += s
            em.persist(s)
        }
        em.flush()
        p.stores.remove(store)
        em.flush()

结果：

Hibernate: insert into product (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: delete from product_stores where product_int=? and stores_int=?

但是使用这个实体（使用List）：

@Entity
class Product(
    @Id
    @GeneratedValue
    val int: Int = 0,
    val name : String,
    @ManyToMany(cascade = [(CascadeType.PERSIST), (CascadeType.MERGE)])
    val stores:  MutableList<Store> = mutableListOf()
)

结果：

Hibernate: insert into product (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into store (name, int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: delete from product_stores where product_int=?
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)
Hibernate: insert into product_stores (product_int, stores_int) values (?, ?)

来自Hibernate用户指南： http://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html#associations-many-to-many

当从@ManyToMany集合中删除实体时，Hibernate只删除链接表中的连接记录。不幸的是，此操作需要删除与给定父级关联的所有条目，并重新创建当前运行的持久化上下文中列出的条目。

但我不知道其背后的原因，为什么我们不能像Set案例一样做

Answer 1

在这里你得到了解释：http://assarconsulting.blogspot.com/2009/08/why-hibernate-does-delete-all-then-re.html

Answer 2

带有List的@ManyToMany注释告诉hibernate期望没有索引的joinTable

在一对多关系中不使用索引列的列表被隐式处理为包，这导致

https://docs.jboss.org/hibernate/stable/core.old/reference/en/html/performance-collections.html

袋子是最糟糕的情况。由于包允许重复的元素值并且没有索引列，因此不能定义主键。 Hibernate无法区分重复的行。 Hibernate通过完全删除（在单个DELETE中）并在每次更改时重新创建集合来解决此问题。

为什么Hibernate在使用List时需要在单向ManyToMany中重新创建其他条目

2 个答案: