[hibernate-issues] [Hibernate-JIRA] Created: (HHH-3930) one-to-one causes redundant select query
one-to-one causes redundant select query
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Key: HHH-3930
URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-3930
Project: Hibernate Core
Issue Type: Bug
Components: core
Affects Versions: 3.3.1
Reporter: Martijn Dashorst
@Entity
public class Address {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@OneToOne(mappedBy = "address")
private Customer customer;
}
@Entity
public class Customer {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
@OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
private Address address = new Address();
public Long getId() {
return id;
}
public Address getAddress() {
return address;
}
}
This mapping causes 2 instead of the expected 1 query to retrieve a Customer and its
Address from the db:
select * from Customer customer0_ left outer join Address address1_ on
customer0_.address_id=address1_.id where customer0_.id=?
select * from Customer customer0_ left outer join Address address1_ on
customer0_.address_id=address1_.id where customer0_.address_id=?
Changing the mapping to a LAZY fetch type:
@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Address address = new Address();
Causes 3 select queries instead of the expected 2 queries to retrieve a Customer (and its
Address) from the db:
select * from Customer customer0_ where customer0_.id=?
select * from Address address0_ left outer join Customer customer1_ on
address0_.id=customer1_.address_id where address0_.id=?
select * from Customer customer0_ where customer0_.address_id=?
The third select is superfluous because the relationship is already completely known: you
already have the customer, so why not just set it on the address entity?
Making the address field in Customer a @ManyToOne doesn't make a difference.
Making the customer field in Address a @OneToMany does remove the extra select, but forces
our model to change the relationship from Customer to List<Customer> where we *know*
there'll be only 1 element.
Apparently Hibernate can figure out the reverse relationship with a @ManyToOne -
@OneToMany without the need for additional queries, can't this be extended to
@OneToOne bidirectional relationships as well?
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