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Hunor Vadasz-Perhat
Hunor Vadasz-Perhat

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hibernate-006: Unidirectional One-to-Many and Many-to-One in Hibernate (Department ↔ Employee)

#java #hibernate #springboot #programming

Unidirectional One-to-Many and Many-to-One using Department and Employee Tables

I'll show you both unidirectional approaches:

  1. Unidirectional @OneToMany → The Department entity knows about Employee, but Employee does NOT know about Department.
  2. Unidirectional @ManyToOne → The Employee entity knows about Department, but Department does NOT know about Employee.

1️⃣ Unidirectional @OneToMany

Entity Definitions 

@Entity public class Department { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @OneToMany @JoinColumn(name = "department_id") // ✅ Forces FK in Employee instead of a join table private List<Employee> employees; }
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@Entity public class Employee { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; }
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2️⃣ Unidirectional @ManyToOne (Recommended)

✅ This is the best practice: The Employee table has a department_id column, and Department does not have a List<Employee>.

Entity Definitions 

@Entity public class Department { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; }
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@Entity public class Employee { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; @ManyToOne @JoinColumn(name = "department_id") // ✅ Foreign key column private Department department; }
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Generated SQL

This creates only two tables, without an extra join table:

CREATE TABLE department ( id BIGINT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(255) ); CREATE TABLE employee ( id BIGINT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(255), department_id BIGINT, FOREIGN KEY (department_id) REFERENCES department(id) );
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✅ This is better because:

  • The foreign key (department_id) is stored inside the Employee table.
  • No extra join table is needed.
  • It’s more efficient for querying.

🔥 Conclusion

Approach Uses @OneToMany? Uses @ManyToOne? Foreign Key in Employee? Extra Join Table? Recommended?
Unidirectional @OneToMany ✅ Yes ❌ No ❌ No ✅ Yes ❌ No
Unidirectional @ManyToOne ❌ No ✅ Yes ✅ Yes ❌ No ✅ Yes

🚀 Best practice: Use only @ManyToOne for a unidirectional relationship.

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