How does the `mappedBy` attribute in JPA and Hibernate help manage bidirectional relationships?

Understanding mappedBy in JPA and Hibernate

Introduction

Bi-directional relationships in object mapping often require careful handling to avoid database inconsistencies. Hibernate offers a concise syntax, mappedBy, to establish these relationships. This article explains the purpose and usage of mappedBy in JPA and Hibernate.

What is mappedBy?

MappedBy specifies that the primary key of the owner entity is stored in a column in the owning table. It allows you to map a bidirectional relationship where only one table has a foreign key to the other. This ensures that the owner entity holds the ownership of the relationship and avoids cyclic relationships.

Purpose of mappedBy

MappedBy solves the issue of avoiding duplicated foreign key constraints in a bi-directional relationship. By designating one table as the owner and storing its primary key in the other table, it ensures consistent data integrity.

Usage

To use mappedBy, you need to annotate the @JoinColumn or @JoinTable in the child entity with the mappedBy attribute. This attribute takes the name of the property in the owner entity that represents the relationship.

Example

Consider the example provided in the query:

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

The @JoinColumn annotation assigns the foreign key constraint to the IDAIRLINE column in the AirlineFlight table. However, the mappedBy attribute is missing.

To make Airline the owner of the relationship, we can modify the annotation as follows:

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

By adding mappedBy="airline", we indicate that the IDAIRLINE column in the AirlineFlight table is the primary key of the Airline entity. This establishes a bidirectional relationship where Airline owns the AirlineFlight instances without the need for a duplicate foreign key in the Airline table.

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