JPA - Basic Projections

In my last post: JPA - Should I become a laziness extremist? - I mentioned about the possibilities of improving JPA usage - one of them is using Projections.

Projection is a subset of entities' properties. It can be represented as dedicated class (or classes), and mapped either as the database view based entity, or using constructor expressions [1][2]. The clue of this solution is having very limited entities tree (or no tree at all, as in my example) comparing to original entity (Employee). We need to display employee name, thus we build the projection having employee name and ID only. As you will see below, using projections leads to single SQL query, instead of bunch of SQL queries (see example in JPA - Should I become a laziness extremist?)

Database view based entities

What we need in this case is simple JPA entity, mapped to any database view (or table if desired properties are in one table).

package com.blogspot.vardlokkur.domain;

import java.io.Serializable;

import javax.annotation.concurrent.Immutable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Immutable
@Table(name = "EMPLOYEE")
public class ViewBasedEmployeeNameProjection implements Serializable {

@Column(name = "EMPLOYEE_ID")
@Id
private Long employeeId;

@Column(name = "EMPLOYEE_NAME")
private String name;

ViewBasedEmployeeNameProjection() {
super();
}

public Long getEmployeeId() {
return employeeId;
}

public String getName() {
return name;
}

}

Now we can run JPQL query: select employee from ViewBasedEmployeeNameProjection employee order by employee.name

package com.blogspot.vardlokkur.domain;

import java.io.Serializable;

import javax.annotation.concurrent.Immutable;

@Immutable
public class EmployeeNameProjection implements Serializable {

private final Long employeeId;

private final String name;

public EmployeeNameProjection(Long employeeId, String name) {
super();
this.employeeId = employeeId;
this.name = name;
}

public Long getEmployeeId() {
return employeeId;
}

public String getName() {
return name;
}

}