Let's define a couple of entities to use in our example. We will create an entity Todo to manage todos. A simple example is shown as follows:

   @Entity
   public class Todo {
     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "userid")
     private User user;
     private String title;
     private String description;
     private Date targetDate;
     private boolean isDone;
     public Todo() {// Make JPA Happy
    }
   }

Important things to note are as follows:

• Todo has a title, a description, a target date, and a completion indicator (isDone). JPA needs a constructor.
• @Entity: The annotation specifies that the class is an entity.
• @Id: Specifies that ID is the primary key of the entity.
• @GeneratedValue(strategy = GenerationType.AUTO): The GeneratedValue annotation is used to specify how the primary key is generated. In this example, we are using a strategy of GenerationType.AUTO. This indicates that we would want the persistence provider to choose the right strategy.
• @ManyToOne(fetch = FetchType.LAZY): Indicates a many-to-one relationship between User and Todo. A @ManyToOne relationship is used on one side of the relationship. FetchType.Lazy indicates that the data can be lazily fetched.
• @JoinColumn(name = "userid"): The JoinColumn annotation specifies the name of the foreign key column.

The following snippet shows the User entity:

   @Entity
   public class User {
     @Id
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
     private String userid;
     private String name;
     @OneToMany(mappedBy = "user")
     private List<Todo> todos;
     public User() {// Make JPA Happy
    }
   }

Important things to note are as follows:

• The user is defined as an entity with the userid and name attributes. The ID is the primary key, which is autogenerated.
• The @OneToMany(mappedBy = "user"): OneToMany annotation is used on the many side of a many-to-one relationship. The mappedBy attribute indicates the property of the owner entity of the relationship.