How does `MappedBy` in JPA and Hibernate help manage bidirectional relationships and improve database schema simplicity?

Understanding MappedBy in JPA and Hibernate

MappedBy plays a crucial role in managing bidirectional relationships in Java Persistence API (JPA) and Hibernate for object-relational mapping. It is a practical way to model and navigate complex relationships in a database context.

Recommended Usage of MappedBy

In your specific example, where Airline owns many AirlineFlights, and many AirlineFlights belong to one Airline, mappedBy is used to avoid repetitive information storage. Since you want AirlineFlights to be the owned class, you specify the mappedBy attribute in the Airline class:

<code class="java">@OneToMany(fetch=FetchType.LAZY, cascade = {CascadeType.ALL})
@JoinColumn(name="IDAIRLINE")
public Set<airlineflight> getAirlineFlights() {
    return airlineFlights;
}</airlineflight></code>

The mappedBy parameter in the @OneToMany annotation specifies that the IDAIRLINE column in the AirlineFlights table is a foreign key that maps back to the primary key in the Airline table. This way, when you query an Airline object, you can also fetch its associated AirlineFlights effortlessly.

Purpose of MappedBy

MappedBy serves several important purposes:

• Prevents SQL Join from Duplicating Information: Without mappedBy, the join between Airline and AirlineFlights would contain both the IDAIRLINE primary key and the foreign key in the AirlineFlights table. This could lead to data duplication and performance issues.
• Improves Database Schema Simplicity: By using mappedBy, you ensure that only one table contains the foreign key, resulting in a simpler and more normalized database schema.
• Supports Bidirectional Navigation: It allows you to traverse the relationships in both directions (from Airline to AirlineFlights and vice versa) without writing explicit join queries.

Your Specific Scenario

In your example, the AirplaneFlights table has the IDAIRLINE column as a foreign key pointing to the Airline table, but the Airline table has no corresponding IDAIRLINEFLIGHTS column. This setup suggests that AirlineFlights is the owning entity, even though your requirement is to have Airline as the owner.

To resolve this, you need to adjust the annotations accordingly. The @OneToMany annotation on airlineFlights should move to the AirlineFlight class, and the mappedBy attribute should refer to the "owning" field in the Airline class:

<code class="java">@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="IDAIRLINE", nullable=false)
@MapsId
public Airline getAirline() {
    return airline;
}</code>

By specifying mappedBy and @MapsId, you indicate that the primary key of Airline (not the IDAIRLINE foreign key in AirlineFlights) is used to define the relationship. This will ensure that Airline is the owning entity and can be fetched eagerly or lazily as needed.

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