Self-Referencing Relationship with Properties in Grails?

Self-Referencing Relationship with Properties in Grails?

I have the following self-referencing relationship. A User can have many
Friends.
class User {
String name
hasMany = [friends: User]
}
What Hibernate does in my MySql database is to create a table user_user.
Now I want to add a property like Date lastUpdated to the friends
relationship. How can I do that?