Unique key auto generated in Spring Boot Entity

user_14157341 0
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I have a primary key in my entity table which is autogenerated but now I want unique keys to be auto generated so how to do it Please help me out.

    @Entity
    @Table(name = "director")
    public class Director {
    
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        @Column(name = "id")
        private long id;
    
//how to make this field auto generated like above one
        @Column(name = "subid", unique=true)
        private long sub_id;

My database table picture is here please refer table

spring-data-jpa spring-boot entity mysql java
2 Answers

You can use timestamp or static AtomicLong counter as sub_id value. Try to define method with annotation @PrePersist in your entity class and your JPA provider will execute it before persisting an object.
Note: using timestamp in concurrent environment may lead to collisions and values won't be unique.

private final static AtomicLong subIdCounter = new AtomicLong(System.nanoTime());

@PrePersist
void sub_id() {
    this.sub_id = subIdCounter.incrementAndGet();
}
user_142245670

After a short study it seems that that Hibernate supports the feature of generated values only with fields annotated with @Id. With @Id and default @generatedValue Hibernate creates - depending on the database and dialect used - appropriate way to generate the value of id field. usually this is something like creating a sequence and setting the column definition like (examples are from Postgres 12):

id bigint not null nextval('director_id_seq'::regclass)

Interesting thing is that this is done by issuing create statement like this:

create table director (id bigserial not null, primary key (id))

So, the column type bigserial actually generates sequence that is used to insert default value to the id column.

There are two options it you want to generate the value for column sub_id as it is generated to the column id. Both are database dependent.

Just create the sequence manually to the database and alter column sub_id to fetch the default value from the sequence.

OR

Change your column definition to use appropriate column type, like:

@Column(name = "subid", insertable = false,
        nullable = false, unique = true, columnDefinition = "bigserial")
private long sub_id;

This will cause Hibernate to generate table like:

create table director (id bigserial not null, subid bigserial not null, primary key (id))

and result to a column like:

subid bigint not null nextval('director_subid_seq'::regclass)

But again: this is database specific stuff.

Also note: that JPA is aware only of the value that is stored to the id field. The subid is inserted to the database table but the sub_id field is not populated until entity is refreshed in its persistence context.

user_64133770
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