globalsetting.rb

class GlobalSetting
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::ForceBoolean

  field :test, type: String, default: "foo bar"

  def self.value(attribute)
    global_setting = first || create!
    global_setting.public_send(attribute)
  end
end

log

[maloik] ~/dev/project  % bin/rails c
Loading development environment (Rails 4.2.3)
irb(main):001:0> @text = GlobalSetting.value(:test)
=> "foo bar"
irb(main):002:0> @text.gsub! /bar/, 'foo'
=> "foo foo"
irb(main):003:0> @text
=> "foo foo"
irb(main):004:0> GlobalSetting.value(:test)
=> "foo foo"