How do you define relationships (OneToMany, ManyToOne)?

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In JPA/Hibernate, relationships between entities are defined using annotations to represent how tables in a database are connected. The most common ones are @OneToMany and @ManyToOne, which describe parent–child relationships.

1. @OneToMany


Used when one entity is related to many others. Example: One Department can have many Employees.

@Entity
public class Department {
    @Id @GeneratedValue
    private Long id;
    private String name;

    @OneToMany(mappedBy = "department", cascade = CascadeType.ALL)
    private List<Employee> employees;
}


Here, mappedBy tells JPA that the relationship is managed by the department field in Employee.

2. @ManyToOne


Used when many entities are related to one parent. Example: Many Employees belong to one Department.

@Entity
public class Employee {
    @Id @GeneratedValue
    private Long id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "department_id")  
    private Department department;
}


The @JoinColumn specifies the foreign key column in the Employee table.

Key Points

Bidirectional: OneToMany + ManyToOne together represent both sides.

Unidirectional: Only one side knows the relationship.

Cascade: Defines what happens to child entities when parent changes (e.g., delete department → delete employees).

Fetch type: LAZY (default for collections) vs EAGER (default for single entity).

👉 In short, @OneToMany defines a parent with multiple children, while @ManyToOne defines children pointing back to one parent. Together, they model relational database associations naturally in JPA.

Would you like me to also cover @OneToOne and @ManyToMany with examples so you have the complete set of relationship mappings?

Read More :


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