Custom field for Rails Devise sign_up action

Question

This is a common question on web, specially here on stack overflow... I know...
anyway I need to perform a little variation on this common rails newbie issue.

I have a little bit complicated situation, then I'm going to semplify it.

On my user model I got all base Devise field, and some custom filed. to manage this custom field on sing_up action I override Registration Controller as the official guide say, and then I add the sign_up_params function as this:

    def sign_up_params
        params.require(:user).permit(:name, :surname, :username, :birthdate, :email, :password, :password_confirmation)
    end

Ok, untill here everything it's going well.

Now I have one more field, that is a reference to another user. One User belong to a parent, and I need to calculate the parent on the server side (should not do this on form, "input type hidden" could create a security issue).

Here the questions:

1- Where's the best place to calculate the parent? on the sign_up_params function like this?

    def sign_up_params
        parent = do_some_stuff_to_retrieve_parent(params[:some_token])
        params.require(:user).permit(:name, :surname, :username, :birthdate, :email, :password, :password_confirmation)
    end

2- When I'll get the parent, how I can put it into the new User object?

EDIT 06/17/15

For more information, here are my user model

    class User < ActiveRecord::Base
      belongs_to :parent, class_name: "User", foreign_key: "parent_id"
      has_many :children, class_name: "User", foreign_key: 'parent_id'
    end

in this situation one child can have only one parent, and one parent can have as much children as he want.
(any correction on model is welcome!)

now I can't write referrer.referrees.create(sign_up_params) but I need to do something like this (really simplified instead of my real code)

    def create
      super
      parent_id = get_parent_id_from_token(params[:token])
      parent = User.find(parent_id)
      @user.parent = parent
    end

whit this code I get this error: can't write unknown attribute parent_id . Where does it come from?

Answer 1

Here's one potential solution. You didn't specify what kind of relationship the two users have, but let's just assume that it's some kind of referral system where an existing user invites a friend with a token.

In the model, you could set up the relationship between users:

class User < ActiveRecord::Base
  belongs_to :referrer, class_name: 'User'
  has_many   :referrees, through: :referrals, class_name: 'User'
end

And then in the controller, you can leverage this relationship to handle setting up the join table entry for you:

class RegistrationsController < Devise::RegistrationsController

  def create
    referrer = User.find_by(token: sign_up_params[:referrer_token])
    referrer.referrees.create(sign_up_params)
    #...
  end

  private

  def sign_up_params
    params.require(:user).permit(:name, :surname, :username, :birthdate, :email, :password, :password_confirmation, :referrer_token)
  end
end

Just a rough idea, haven't tested the code or anything :)