hibernate unique constraint [closed]

Closed. This question needs details or clarity. It is not currently accepting answers. 开发者_StackOverflow社区

---

Want to improve this question? Add details and clarify the problem by editing this post.

Closed 4 years ago.

Improve this question

I was faced with the below problem problem

P.S. Have a look at the comments :) on the above link

And while solving it, I now have a question about how the unique constraint is implemented in hibernate, I have started to believe that it fires a select query (I am not sure about this), and then "some how" performs the validation.

I am not very convinced with the explanation

---

Hibernate creates a "unique" index on that column, and it's the database that then enforces the uniquness.

For example, if you have a class:

@Entity
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
public class UserEmailAddressEntity {
    @Id
    @Basic(optional = false)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Basic(optional = false)
    private boolean primaryEmail;

    @ManyToOne(optional = false)
    private UserEntity user;

    @NaturalId // this means the email column must be unique
    @Basic(optional = false)
    private String email;

    @Basic(optional = false)
    private String token;

    @Basic(optional = false)
    private boolean verified;
}

Hibernate creates a table like so: (for PostgreSQL, but the idea is the same for pretty much all RDBMS)

CREATE TABLE useremailaddressentity
(
  id bigint NOT NULL,
  email character varying(255) NOT NULL,
  primaryemail boolean NOT NULL,
  token character varying(255) NOT NULL,
  verified boolean NOT NULL,
  user_id bigint NOT NULL,
  CONSTRAINT useremailaddressentity_pkey PRIMARY KEY (id),
  CONSTRAINT fk279a5e06c843ec30 FOREIGN KEY (user_id)
      REFERENCES userentity (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,

  -- note the `UNIQUE` here:
  CONSTRAINT useremailaddressentity_email_key UNIQUE (email)
)

---

I tried to replicate this behavior , I using using grails 1.3.7 and found it to be reproducible

class Child {
    String name
    static constraints = { name(unique:true) }
}

table created 
CREATE TABLE `child` (
  `id` BIGINT(20) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(255) NOT NULL,  
  PRIMARY KEY  (`id`),
  UNIQUE KEY `name` (`name`)
)

Queries fired on child.save()

Hibernate: select this_.id as id0_0_, this_.version as version0_0_, this_.created_by as created3_0_0_, this_.date_created as date4_0_0_, this_.last_updated as last5_0_0_, this_.name as name0_0_, this_.updated_by as updated7_0_0_ from child this_ where this_.name=?
Hibernate: insert into child (version, created_by, date_created, last_updated, name, updated_by) values (?, ?, ?, ?, ?, ?)

The reason i think i hibernate fires the above query is to check the unique constraint, And in case you are trying to perform an update then this query will result in having another object with the same identifier in memory, which could the lead to nonuniqueobjectexception.

I thinking this is hibernate and not grails, have not double checked this in java/hibernate.

Thanks