Inheritance in JPA/Hibernate

@MappedSuperclass

• Description: A class-level annotation applied to an abstract or concrete superclass. Hibernate does not generate a table for this class.
• Purpose: Defines common fields inherited by entity classes.
• Common Fields:
  • ID: Primary key (e.g., Long id with @Id).
  • Creation Date/Time/Timestamp: @CreationTimestamp for tracking entity creation.
  • Update Date/Time/Timestamp: @UpdateTimestamp for tracking entity updates.
  • Version: @Version private int version for optimistic locking (optional, used to manage concurrent updates).

Example:

import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.MappedSuperclass;
import jakarta.persistence.Version;
import org.hibernate.annotations.CreationTimestamp;
import org.hibernate.annotations.UpdateTimestamp;
import java.time.LocalDateTime;

@MappedSuperclass
public abstract class BaseEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @CreationTimestamp
    private LocalDateTime createdAt;

    @UpdateTimestamp
    private LocalDateTime updatedAt;

    @Version
    private int version; // For optimistic locking

    // Getters and Setters (or use Lombok @Getter/@Setter)
}

Bidirectional Association: Category ↔ Product

• Relationship: Category (one, parent, inverse) ↔ Product (many, child, owning, contains FK).
• Context: Similar to the Restaurant ↔ FoodItem relationship in the Food Delivery App from previous queries.

Entity Configurations

Category Entity

import jakarta.persistence.Entity;
import jakarta.persistence.OneToMany;
import jakarta.persistence.CascadeType;
import java.util.ArrayList;
import java.util.List;
import lombok.Getter;
import lombok.Setter;
import lombok.NoArgsConstructor;
import lombok.AllArgsConstructor;
import lombok.ToString;

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString(exclude = {"products"}, callSuper = true)
public class Category extends BaseEntity {
    private String name;
    private String description;

    @OneToMany(mappedBy = "productCategory", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Product> products = new ArrayList<>();

    // Helper methods (per Gavin King’s recommendation)
    public void addProduct(Product product) {
        products.add(product);
        product.setProductCategory(this);
    }

    public void removeProduct(Product product) {
        products.remove(product);
        product.setProductCategory(null);
    }
}

Product Entity

import jakarta.persistence.Entity;
import jakarta.persistence.ManyToOne;
import jakarta.persistence.JoinColumn;
import lombok.Getter;
import lombok.Setter;
import lombok.NoArgsConstructor;
import lombok.AllArgsConstructor;
import lombok.ToString;

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString(callSuper = true)
public class Product extends BaseEntity {
    private String name;
    private double price;

    @ManyToOne
    @JoinColumn(name = "category_id", nullable = false)
    private Category productCategory;
}

Notes on Configuration

• Inheritance: Both Category and Product extend BaseEntity, inheriting id, createdAt, updatedAt, and version.
• Bidirectional Association:
  • Category is the inverse (non-owning) side, using mappedBy = "productCategory".
  • Product is the owning side, containing the foreign key (category_id).
• Cascading: cascade = CascadeType.ALL propagates operations (e.g., persist, merge, remove) from Category to Product.