当我删除@OneToMany 关系中的实体时，Hibernate，n+1 查询

Question

I want to define a bidirectional @OneToMany relation. I did it the following way:

public class Customer {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private long id;

  @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL, orphanRemoval = true)
  @JsonIgnore
  @JsonProperty(access = JsonProperty.Access.WRITE_ONLY)
  private List<CustomerEmployeeRole> roles;

  // more fields
}

public class CustomerEmployeeRole {

  @ManyToOne
  @NotNull
  private Customer customer;

 // more fields
}

Everything works as expected, the only problem is that n+1 queries are executed when i delete a customer. n is the number of roles the customer has. For every role the following query is executed:

delete from customer_employee_role where id=?

It would be possible to delete all roles in one query:

delete from customer_employee_role where customer_id=?

I assume hibernate does n+1 queries because of mappedBy = "customer" . I want to keep the annotation, because i want to avoid a join table. Is there any way to tell hibernate to only execute one query instead of n queries in this case? Otherwise i would have to write my own queries which would work too, but it´s not really elegant.

Answer 1

The n+1 queries are because of the CascadeType.ALL . Since you set this attribute, the CustomerEmployeeRole's are deleted too. So if you want to remove a Customer let's see what happens assuming that the customer have 2 roles:

1) delete from CustomerEmployeeRole where id =..

2) delete from CustomerEmployeeRole where id =..

3) delete from Customer where id =.. here is your +1 query.

PS: In case you have any more questions about which is the best way to map a @OneToMany , @ManyToMany , @OneToOne or what is happening behind the scenes this is a good tutorial. ( Same for the other annotations )