Grails/Groovy: Cannot cast object to java.util.Set (twitter clone project)

Question

I am trying to make a follow user like in twitter. I'm getting an exception when I try to save the user of the currently logged in session user.

Error 500: Internal Server Error
URI - /blog-dwit/user/follow/3
Class -  org.codehaus.groovy.runtime.typehandling.GroovyCastException
Message - Cannot cast object 'blog.dwit.User : 3' with class 'blog.dwit.User' to class 'java.util.Set'

User domain:

class User {
    String email_id;
    String password;
    Profile profile

    static hasMany = [ posts:Post, following: User ]
}

Controller action

 def follow(){
    def followUser = User.get(params.id)
    def user = User.get(session.user)
        user.following = followUser
        user.save()

}

Answer 1

When adding instances to collections you have to use the addTo* method for the corresponding collection. The documentation explains this in further detail.

Your action should look like this:

 def follow(){
    def followUser = User.get(params.id)
    def user = User.get(session.user)

    user.addToFollowing(followUser)
    user.save()
 }