Ids

Primary key

A primary key is identified on an entity using the Spring @Id annotation.

@Table
public class Entity {

  @Id
  private Long id;

}

A primary key may be any type supported by the database (numbers, string, UUID, dates...).

Generated primary key

An additional @GeneratedValue annotation can be used to ask the database to automatically generate the value:

@Table
public class Entity {

  @Id @GeneratedValue
  private Long id;

}

There are 3 generation strategies available, using the strategy attribute on the annotation:

• AUTO_INCREMENT (default) to automatically increment the value
• SEQUENCE to use a database sequence to generate the next value. The sequence name must be specified using the sequence attribute on the annotation
• RANDOM_UUID to generate a random UUID if supported by the database (PostgreSQL needs to configure the extension uuid-ossp).

Example for a sequence:

@Table
public class Entity {

  @Id @GeneratedValue(strategy = Strategy.SEQUENCE, sequence = "my_sequence")
  private Long id;

}

Example for a random UUID:

@Table
public class Entity {

	@Id @GeneratedValue(strategy = Strategy.RANDOM_UUID)
	private UUID id;

}

Composite id

When several columns must be used as primary key, the @CompositeId annotation can be used, specifying the 2 attributes:

• indexName name of the index to create on the columns (used only for schema generation)
• properties name of attributes on the entity class to use

Here is an example with a table population where we want to use the date and the city as primary key:

@Table
@CompositeId(indexName = "population_key", properties = { "date", "city" })
public class Population {

	@Column
	private LocalDate date;
	
	@Column
	private String city;
	
	@Column
	private Long population;

	[...]
}

Note: Any type of column may be used, including foreign keys.

Next

Next step is to link tables, with one-to-one, one-to-many and many-to-many Relationships.