Why You Should Avoid the Unidirectional @OneToMany Association in JPA

When designing a database schema in JPA, relationships between entities play a crucial role in ensuring performance, maintainability, and readability. A common decision is how to model a one-to-many relationship. While a unidirectional @OneToMany association may seem like a straightforward approach, it can lead to significant inefficiencies compared to its bidirectional counterpart.

Here we explore the drawbacks of unidirectional @OneToMany associations, particularly focusing on performance penalties during database operations.

Let’s consider the example of Company and Employee entities:

• A Companycan have multiple Employee(one-to-many).
• Each Employeeis associated with a single Company(many-to-one).

Unidirectional

//Now, let’s assume that the Company and Employee entities are involved in a unidirectional @OneToMany association mapped, as follows:

@OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
private List<Employee> employees = new ArrayList<>();

Bidirectional

public class Company {
    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Employee> employees= new ArrayList<>();
}

public class Employee {
    @ManyToOne
    @JoinColumn(name = "company_id")
    private Company company;

}

//Reference is mentioned in both the classes

In a bidirectional lazy @OneToMany association, JPA efficiently handles this relationship by keeping the foreign key column in the Employeetable. Operations like inserting or deleting a Employeeare simple: they require a single INSERT or DELETE operation directly in the Employeetable.

However, with a unidirectional @OneToMany association, where Companyreferences Employeewithout a corresponding @ManyToOne mapping in Employee, JPA introduces a junction table to manage the association. This junction table contains foreign keys pointing to both Companyand Employee. While this design eliminates direct back-references, it comes with significant downsides.

Performance Issues in Unidirectional @OneToMany

1. Increased Database Overhead