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Module 3: Entity Relationships (The Connected Shelves)

📚 Module 3: Entity Relationships

Focus: Moving from “Single Tables” to “Connected Systems.”

In the real world, data is rarely alone. A Category has many Products. A Customer has many Orders. In JPA, we use Annotations to connect our shelves together.

This is the most common relationship. One “Parent” owns many “Children.”

🧩 The Analogy: The Folder and the Files

  • One Folder (The Parent) can contain 100 Files (The Children).
  • If you delete the Folder, you probably want to delete the Files too (Cascading).

In Code:

@Entity
public class Category {
    @Id
    private Long id;
    private String name;

    @OneToMany(mappedBy = "category")
    private List<Product> products; // One category has many products
}

🏗️ Step 2: Many-to-One (The “Reference”)

This is the opposite side of the relationship. Every child points back to its parent.

🧩 The Analogy: The Store Tag

  • Every Product (The Child) has a tag that says: “I belong to the Electronics Category.”
  • Many products can have the same tag.

In Code:

@Entity
public class Product {
    @Id
    private Long id;

    @ManyToOne
    @JoinColumn(name = "category_id") // The SQL foreign key
    private Category category;
}

🏗️ Step 3: Cascading & Fetching (The “Delivery”)

  1. CascadeType.ALL: If I save the Parent, automatically save all the Children.
  2. FetchType.LAZY: Don’t load the Children from the database until I specifically ask for them. (This saves memory!).

🥅 Module 3 Review

  1. @OneToMany: One parent, many children.
  2. @ManyToOne: Many children pointing to one parent.
  3. mappedBy: Tells Spring who is the “Boss” of the relationship.
  4. JoinColumn: The actual column in SQL that holds the connection (Foreign Key).