How to create unique IDs for child entities containing Key fields?

Question

In the demo code below, the User is the parent entity, with a one-to-many relationship with a BlogPost.

Because the BlogPost is a child of a User, its primary key field cannot be an auto-generated long value but has to be a Key object instantiated with the KeyFactory.

At the moment, I am passing the BlogPost blogTitle (not guaranteed unique) to the createKey method when instantiating a Key for the BlogPost

But I don't know how to generate a unique ID value for the BlogPost?

@Entity
public class BlogPost {

    @Id
    private Key key;
    private String blogTitle;
    private Text blogText;

    @ManyToOne(fetch = FetchType.LAZY)
    private User user;

    public BlogPost(Key parentKey, String blogTitle) {
      this.key = KeyFactory.createKey(parentKey, "BlogPost", blogTitle);
    }
}

@Entity
public class User {

    @Id
    private Key key;
    private String username;
    private String email;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<BlogPost> blogPosts;

    public User(String username){
      this.key = KeyFactory.createKey("User", username);
    }
}

Answer 1

You can still use an auto-generated key; just use this:

@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

IDs generated this way will be unique for a given parent entity, so to uniquely identify a post, you'll need the ID or name of the User object, and the ID of the BlogPost.