Question

I currently have a table which looks like this:

Users

username
address
dob
…

And

Roles
username
role

I want to make a foreign key constraint between Role.username and User.username. How should I go about doing this with ann

Answer 1

1. Would be good if you could drop the “s” from the end of your table/type names – each row/instance represents one item, not many.

2. Use @ManyToOne annotated field within Role entity – with field of type “User” and name “user”. This is a relationship field that is owned by Role – the FK column will go in Role table.

3. Optionally, use @OneToMany(mappedBy="user") annotation field within User entity – with field of type Collection<Role> or List<Role>, dependending on whether you want to preserve the ordering of app insertion when you write to DB. This is a non-owned relationship field, meaning no FK column will go into User table – we must use mappedBy to name the owned relationship on the User entity. However, if you omit (2) and include (3) this becomes owned, and then mappedBy is not used, but column=”some_db_colname” could be used instead.

4. In User entity set @Id on username (single PK field of basic type).

5. In Role entity set @Id on both user (non-basic type) and also role (basic type).

Additionally, because you have two @Id fields and because one of them is not a basic type, you must additionally create a RoleId class that repeats the same field names, but this time, maps the data types to the underlying basic types – as found in the PK field of the referenced entities and as consistent with underlying DB table. Annotate Role with @IdClass(RoleId)

@Entity
public class User {

     @Id
     String username;  // for simplicity match the db column name

     String address;

     Date dateOfBirth;

     @OneToMany(mappedBy="user")
     Collection<Role> rolesForUser;

     // other fields and methods
}

@Entity
@UserId(RoleId)
public class Role {

     @Id
     @ManyToOne
     User user;

     @Id
     String role;  // for simplicity, match the DB column name

     // other fields and methods
}

public class RoleId {

     String user;

     String role;

     public RoleId() { // ...}  // all entities must have no-arg constructor
     // additional arg constructor - so we can set fields without having setters
     public RoleId(String user, String role) { this.user = user; this.role = role}  

     public String getUser() { return user; }  // omit setter: Id is immutable once created
     public String getRole() { return role; }  // omit setter: Id is immutable once created
}

Ready to use… =:-)