Many-One Relationship

@OneToMany

This annotation is used to define a one-to-many relationship between two entities.
It’s typically used on the “one” side of the relationship.

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

@ManyToOne

This annotation is used to define a many-to-one relationship between two entities.
It’s typically used on the “many” side of the relationship.

public class Employee {
    @ManyToOne
    @JoinColumn(name = "department_id")
    private Department department;
}

Often, @OneToMany and @ManyToOne are used together to create a bidirectional relationship. The @OneToMany side uses mappedBy to indicate the field that owns the relationship. The @ManyToOne side use @JoinColumn to specify the foreign key column.

Cascade operation

Cascade operations in ORM frameworks like Hibernate and JPA allow you to automatically apply operations performed on a parent entity to its associated child entities. This feature is particularly useful in managing relationships between entities and can significantly simplify database operations.

Types

PERSIST: Saves the child entity when the parent is saved.
MERGE: Updates the child entity when the parent is updated.
REMOVE: Deletes the child entity when the parent is deleted.
REFRESH: Refreshes the child entity when the parent is refreshed.
DETACH: Detaches the child entity when the parent is detached (from the persistence context).
ALL: Applies all cascade types (PERSIST, MERGE, REMOVE, REFRESH, DETACH).

Usage

You can specify cascade operations in the @OneToMany, @ManyToOne, @OneToOne, or @ManyToMany annotations:

@OneToMany(cascade = CascadeType.ALL)
private List<ChildEntity> children;

@OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
private List<ChildEntity> children;

Considerations

Orphan Removal: Used in conjunction with cascading, orphanRemoval = true will remove child entities that are no longer referenced by the parent.
Bi-directional Relationships: Be cautious with cascade operations in bi-directional relationships to avoid unintended side effects.
Performance: Cascading can impact performance, especially with large datasets. Use it judiciously