Mappings

• One-to-One

• E.g., a instructor has its details in a separate table

• Use @OneToOne and @JoinColumn(name = fk_field)

• Use @OneToOne(mappedBy=instructorDetail) for a bidirectional (when the first unidirectional reference has already been made). This uses the instructorDetail property of Instructor class and use its information of @JoinColumn

• One-to-Many (Many-to-One)

• E.g., a instructor can have multiple courses

• One-to-Many relationships can be unidirectional or bidirectional

• Do not use cascade delete if the reference is bidirectional

• Many-to-Many

• E.g., a course can have many students, students can have many courses

• Never use cascade delete on many to many relationships

Cascade Types

• Cascade is the action of applying the same operation to related entities
• Cascading finds the related table by the primary key and foreign key

• By default, no operations are cascaded

• Cascade Types

• Persist: if entity is saved, related entity will also be saved
• Remove: if entity is removed, related entity will also be removed
• Refresh: if entity is refreshed, related entity will also be refreshed
• Detach: if entity is detached, related entity will also be detached
• Merge: if entity is merged, related entity will also be merged
• All: all of the above cascade types

@Entity
@Table(name="instructor")
public class Instructor {
    @OneToOne(cascade = CascadeType.All) // cascade all operations to the relatedentities
    //@OneToOne(cascade = {CascadeType.DETACH, CascadeType.MERGE}) // list of cascade types
    @JoinColumn(name = "instructor_detail_id") // primary key
    private InstructorDetail instructorDetail;
}

Data fetch

• Eager: retrieve everything from all related tables in one shot
• Lazy: retrieve the data from the related tables only when requested (on demand). Preferred!
• @OneToMany(mappedby = "instructor", fetch = FetchType.LAZY)
• To retrieve data at a latter time, the hibernate session must be opened (otherwise it throws an exception - LazyInitializationException).

Mapping	Default Fetch Type
@OneToOne	FetchType.EAGER
@OneToMany	FetchType.LAZY
@ManyToOne	FetchType.EAGER
@ManyToOne	FetchType.LAZY

Join Table

• Join Table is a table that provides a mapping between 2 tables in a Many To Many relationship
• It has a foreign key for each of both tables.
• The foreign keys are also the primary key of the join table
• @JoinTable is used for Many to Many relationships

• Join Column
• Inverse Join Column Inverse