How Does `MappedBy` Facilitate Bidirectional Relationships in JPA and Hibernate?

Understanding MappedBy in JPA and Hibernate

In object-relational mapping (ORM) frameworks like JPA and Hibernate, MappedBy is a useful annotation for establishing bidirectional relationships between entity classes. This annotation is used in the context of one-to-many and many-to-one relationships.

Recommended Usage

The recommended usage of MappedBy is as follows:

• One-to-Many Relationships: In a one-to-many relationship, the "many" side of the relationship (the side with a collection) should have the MappedBy annotation. This annotation specifies the name of the field in the "one" side of the relationship that holds the foreign key constraint.
• Many-to-One Relationships: In a many-to-one relationship, the "one" side of the relationship (the side with a single referencing object) should have the MappedBy annotation. This annotation specifies the name of the field in the "many" side of the relationship that holds the foreign key constraint.

Purpose of MappedBy

MappedBy serves several purposes:

• Bi-Directional Traversal: In bidirectional relationships, MappedBy allows you to traverse the relationship in both directions without the need for explicit foreign key definitions in both tables.
• Optimization: By specifying the ownership of the relationship, MappedBy can optimize database performance by avoiding unnecessary join tables or foreign key constraints.

Example

In your provided example, Airline owning many AirlineFlights and AirlineFlights belonging to one Airline, MappedBy should be used in the AirlineFlight class as follows:

<code class="java">@Entity
@Table(name="AirlineFlight")
public class AirlineFlight {
    // ... other attributes ...

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="IDAIRLINE", nullable=false)
    @MapsId
    private Airline airline;
    // ... other attributes ...
}</code>

In this example, the @MapsId annotation indicates that the IDAIRLINE field in AirlineFlight is used as the foreign key constraint to the Airline entity. By using MappedBy here, you can establish a bidirectional relationship between Airline and AirlineFlight without the need for foreign key constraints in both tables.

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