🚀 Can You Mix Unidirectional and Bidirectional @OneToMany & @ManyToOne?

✅ Yes, you can! You can choose:

• @ManyToOne without @OneToMany → Unidirectional
• @ManyToOne with @OneToMany(mappedBy) → Bidirectional

📌 Mixing Unidirectional & Bidirectional Approaches

1️⃣ Example: @ManyToOne Without @OneToMany (Unidirectional)

✅ Best for queries from Employee → Department but NOT the other way around.

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "department_id") // ✅ Foreign key in Employee table
    private Department department;
}

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
}

✅ Queries:

• employee.getDepartment(); (Works ✅)
• department.getEmployees(); (Not possible ❌ - No @OneToMany)

🚀 Use case: If you only need to retrieve an employee's department, but NOT employees from a department.

2️⃣ Example: @ManyToOne + @OneToMany(mappedBy) (Bidirectional)

✅ Best if you need queries in both directions.

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "department_id") // ✅ Foreign key in Employee table
    private Department department;
}

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "department") // ✅ Refers to Employee.department
    private List<Employee> employees;
}

✅ Queries:

• employee.getDepartment(); (Works ✅)
• department.getEmployees(); (Works ✅)

🚀 Use case: If you need to retrieve both:

• All employees in a department
• An employee’s department

🎯 Final Recommendation

✅ Best Practice:

• Use @ManyToOne by itself (Unidirectional) if querying only from child → parent.
• Use @ManyToOne + @OneToMany(mappedBy) (Bidirectional) if querying both ways.