Working with JPA Entity Relationships

@Entity class User {
  @Id @GeneratedValue Long id;
  @OneToOne(mappedBy="user", cascade=CascadeType.ALL, orphanRemoval=true)
  Profile profile;
}
@Entity class Profile {
  @Id @GeneratedValue Long id;
  @OneToOne(fetch=FetchType.LAZY)
  @JoinColumn(name="user_id", unique=true)
  User user;
}

@Entity class LineItem {
  @Id @GeneratedValue Long id;
  @ManyToOne(fetch=FetchType.LAZY, optional=false)
  @JoinColumn(name="order_id", nullable=false)
  Order order;
}

@Entity class Course {
  @ManyToMany
  @JoinTable(name="course_student",
    joinColumns=@JoinColumn(name="course_id"),
    inverseJoinColumns=@JoinColumn(name="student_id"))
  Set<Student> students = new HashSet<>();
}

@ManyToOne
@JoinColumn(name="customer_id",
            referencedColumnName="id",
            nullable=false,
            foreignKey=@ForeignKey(name="fk_order_customer"))
Customer customer;

@OneToMany(mappedBy="order", cascade=CascadeType.ALL, orphanRemoval=true)
List<LineItem> items;
// order.getItems().remove(li); // triggers DELETE

@EntityGraph(attributePaths = {"customer", "items.product"})
@Query("select o from Order o where o.id = :id")
Optional<Order> findByIdWithDetails(@Param("id") Long id);

// Or named graph
@NamedEntityGraph(name="Order.detail",
  attributeNodes = {@NamedAttributeNode("items"), @NamedAttributeNode("customer")})