Entity Relationships

Introduction

Relationships model how entities reference each other. Get the ownership and fetch strategy right, or you'll fight Hibernate forever.

Understanding the topic

The four mappings, owners marked with ★:

• @OneToOne — User ★ ↔ Profile. Owner has the FK.
• @OneToMany / @ManyToOne — Order ↔ ★ Item. The many side owns the FK. Always.
• @ManyToMany — Student ↔ Course via a join table. Avoid in real apps; model the join as a first-class entity.

Informative example

@Entity
public class Order {
  @Id @GeneratedValue Long id;

  @OneToMany(
      mappedBy = "order",                 // 'order' field on OrderItem owns the FK
      cascade = CascadeType.ALL,
      orphanRemoval = true,
      fetch = FetchType.LAZY
  )
  private List<OrderItem> items = new ArrayList<>();
}

@Entity
public class OrderItem {
  @Id @GeneratedValue Long id;

  @ManyToOne(fetch = FetchType.LAZY, optional = false)
  @JoinColumn(name = "order_id")
  private Order order;
}

Best practices

• Own the FK on the many side; mappedBy on the one.
• Default fetch to LAZY on every relation.
• Replace @ManyToMany with a join entity — you'll need extra columns soon enough.