Hibernate @AssociationOverride for References Override in Subclass

@AssociationOverride is pivotal when an entity inherits mappings from a superclass, but you need to tweak the association details in the subclass. Let's delve into its utility and implications through a practical example involving geometric figures and colors.

Hibernate @AssociationOverride annotation for References Override

Hi, this is Paul, and welcome to this article, we explore how to override REFERENCES in the Hibernate entity in a subclass.

Introduction to the Scenario

Our example involves two geometric entities: Circle and Square, both inheriting from a Figure superclass. Both shapes are associated with a color, but the foreign key column linking to the color differs in each subclass's table. Here's how our database schema looks:

• Colors Table: Holds color data.
• Circles Table: Represents circles, linked to colors.
• Squares Table: Represents squares, also linked to colors.

CREATE TABLE colors
(
    id   BIGSERIAL PRIMARY KEY,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE circles
(
    id              BIGSERIAL PRIMARY KEY,
    circle_color_id BIGINT,
    radius          DOUBLE PRECISION NOT NULL,
    FOREIGN KEY (circle_color_id) REFERENCES colors (id)
);

CREATE TABLE squares
(
    id              BIGSERIAL PRIMARY KEY,
    square_color_id BIGINT,
    side_length     DOUBLE PRECISION NOT NULL,
    FOREIGN KEY (square_color_id) REFERENCES colors (id)
);

MappedSuperclass: Setting the Stage

Figure is annotated as a @MappedSuperclass, meaning it provides mapping information to its subclasses but is not an entity itself.

Hibernate @MappedSuperclass for Inheritance

This approach is useful when you want common mappings (like id and color) but do not need a table for the superclass.

import jakarta.persistence.*;
import lombok.Getter;
import lombok.Setter;

@Getter
@Setter
@MappedSuperclass
public abstract class Figure {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "color_id")
    protected Color color;
}

@AssociationOverride: Customizing Mappings

The subclasses Circle and Square override the color association using @AssociationOverride. This is crucial because while they inherit the color mapping from Figure, the actual foreign key column in their respective tables is different (circle_color_id for Circle and square_color_id for Square).

import jakarta.persistence.*;
import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@DiscriminatorValue("Square")
@AssociationOverride(
        name="color", 
        joinColumns=@JoinColumn(name="square_color_id")
)
@Table(name = "squares")
public class Square extends Figure {
    @Column(name = "side_length")
    private double sideLength;
}

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@DiscriminatorValue("Circle")
@AssociationOverride(
        name="color", 
        joinColumns=@JoinColumn(name="circle_color_id")
)
@Table(name = "circles")
public class Circle extends Figure {
    @Column(name = "radius")
    private double radius;
}

How @AssociationOverride Works

@AssociationOverride allows you to redefine how an inherited association is mapped in a subclass. In our case, although both subclasses inherit the color field from Figure, each needs it linked to a different column. Without this override, Hibernate would look for a color_id column by default, as specified in the Figure class.

Conclusion

@AssociationOverride is a testament to Hibernate's flexibility in handling complex inheritance structures. By mastering this annotation, developers can ensure their data access layer is both robust and aligned with the nuanced requirements of their domain model. Understanding and leveraging such annotations enhances the maintainability and scalability of applications, allowing them to evolve gracefully alongside business needs.

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Paul Ravvich