1. Revise

Unidirectional One-to-One

• Definition: A relationship where one entity is associated with exactly one instance of another entity, navigable in one direction only.

• Example: Product HAS-A ProductDetails (1:1).

  • Product has a reference to ProductDetails, but ProductDetails does not reference Product.

• Annotations:

  • @OneToOne on the owning side (Product).
  • @JoinColumn to specify the foreign key column.

• Code Example:

  @Entity
  public class Product {
      @Id
      private Long id;
      private String name;
      @OneToOne
      @JoinColumn(name = "product_details_id")
      private ProductDetails productDetails;
      // Getters, setters
  }
  
  @Entity
  public class ProductDetails {
      @Id
      private Long id;
      private String description;
      // No reference to Product
      // Getters, setters
  }
  
  

• Database: Product table has a product_details_id column (foreign key to ProductDetails).

Unidirectional Many-to-Many

• Definition: A relationship where multiple instances of one entity can be associated with multiple instances of another, navigable in one direction.

• Example: Product HAS-A multiple Brands, and vice versa (:), but only Product references Brands.

• Annotations:

  • @ManyToMany on the owning side (Product).
  • @JoinTable to define the join table, specifying joinColumns (for Product) and inverseJoinColumns (for Brand).

• Code Example:

  @Entity
  public class Product {
      @Id
      private Long id;
      private String name;
      @ManyToMany
      @JoinTable(
          name = "product_brand",
          joinColumns = @JoinColumn(name = "product_id"),
          inverseJoinColumns = @JoinColumn(name = "brand_id")
      )
      private List<Brand> brands;
      // Getters, setters
  }
  
  @Entity
  public class Brand {
      @Id
      private Long id;
      private String name;
      // No reference to Product
      // Getters, setters
  }
  
  

• Database: A join table product_brand with columns product_id and brand_id.

2. JPA Components (Value Types) – Embeddables

• Definition: Value types (like Embeddable classes) lack their own database identity and are embedded within an owning entity. Covered in your previous query.
• Key Points:
  • @Embeddable: Marks a class as a value type (e.g., Address).
  • @Embedded: Used in the entity to include the embeddable.
  • No shared references; each entity has its own copy of the value type.
  • Example: Address embedded in User or College.

3. Enter Spring

What is Spring?