What is the \'Owning Side\' in Object-Relational Mapping (ORM)?

Understanding the "Owning Side" in ORM Mapping

In Object-Relational Mapping (ORM), the concept of "owning side" refers to the entity that manages the foreign key relationship in a database. This arises because relational databases do not inherently support bidirectional relationships, unlike object-oriented programming.

Necessity of an Owning Side

In ORM, the owning side is responsible for maintaining the integrity of a relationship by owning the foreign key that represents the other entity in the relationship. Without an owning side, each side of the relationship would require a foreign key, leading to unnecessary association tables and potential data duplication.

Ownership in Different Relationship Types

• One-to-Many: The owning side typically contains the foreign key in the child table, allowing multiple children to be associated with a single parent.
• One-to-One: Similar to one-to-many, the owning side holds the foreign key, while the non-owning side uses the mappedBy attribute to indicate the relationship.
• Many-to-One: The owning side holds the foreign key, allowing multiple entities to reference a single parent.

Example: One-to-Many Mapping with Owning Side

Consider the following Java example:

@Entity
public class Person {
    @Id
    private Long id;
    @OneToMany(mappedBy = "person")
    private List<IdDocument> idDocuments;
}

@Entity
public class IdDocument {
    @Id
    private Long id;
    @ManyToOne
    private Person person;
}

In this example, Person is the owning side, as it contains the foreign key that references the IdDocument table. The mappedBy attribute in IdDocument indicates that the relationship is managed by the owning side.

Implications of Using mappedBy

Using mappedBy on the non-owning side shifts the responsibility of managing the relationship to the owning side. This means:

• The foreign key updates must only be performed on the owning side (e.g., idDocument.setPerson(person)).
• Hibernate will not track changes to the relationship on the non-owning side (e.g., person.getIdDocuments().add(idDocument)).

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