How to initialize field based on another field?

Question

I have a user model with :email and :user_name, I want to auto initialize :user_name with part of :email.

class User
  include Mongoid::Document
  field :email
  field :user_name
end

I use simple_form to create the user. How can I initialize :user_name based on :email?

Answer 1

In your model:-

before_create :add_user_name

private
  def add_user_name
    self.user_name = self.email
  end

Answer 2

Use default:

class User
 include Mongoid::Document
 field :email, type: String
 field :user_name, default: ->{ "email_is: #{email}" }
end

"If you want to set a default with a dependency on the document's state, self inside a lambda or proc evaluates to the document instance."

See: Mongoid fields documentation

Tested with rails c:

u = User.new email: "name@domain.com"
u.user_name # => "email_is: name@domain.com"