Question - What is Single Table Strategy?
Answer -
Single Table Strategy is a hibernate’s strategy for performing inheritance mapping. This strategy is considered to be the best among all the other existing ones. Here, the inheritance data hierarchy is stored in the single table by making use of a discriminator column which determines to what class the record belongs.
For the example defined in the Hibernate Inheritance Mapping question above, if we follow this single table strategy, then all the permanent and contract employees’ details are stored in only one table called InterviewBitEmployee in the database and the employees would be differentiated by making use of discriminator column named employee_type.
Hibernate provides @Inheritance annotation which takes strategy as the parameter. This is used for defining what strategy we would be using. By giving them value, InheritanceType.SINGLE_TABLE signifies that we are using a single table strategy for mapping.
@DiscriminatorColumn is used for specifying what is the discriminator column of the table in the database corresponding to the entity.
@DiscriminatorValue is used for specifying what value differentiates the records of two types.
The code snippet would be like this:
InterviewBitEmployee class:
@Entity
@Table(name = "InterviewBitEmployee")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "employee_type")
@NoArgsConstructor
@AllArgsConstructor
public class InterviewBitEmployee {
@Id
@Column(name = "employee_id")
private String employeeId;
private String fullName;
private String email;
}
InterviewBitContractEmployee class:
@Entity
@DiscriminatorValue("contract")
@NoArgsConstructor
@AllArgsConstructor
public class InterviewBitContractEmployee extends InterviewBitEmployee {
private LocalDate contractStartDate;
private LocalDate contractEndDate;
private String agencyName;
}
InterviewBitPermanentEmployee class:
@Entity
@DiscriminatorValue("permanent")
@NoArgsConstructor
@AllArgsConstructor
public class InterviewBitPermanentEmployee extends InterviewBitEmployee {
private LocalDate workStartDate;
private int numberOfLeaves;
}