Use extend module Settings or reuse one method from settings model

add_setting.rb

require 'active_record'

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

ActiveRecord::Schema.define do
  self.verbose = false

  create_table :settings do |t|
    t.string :name
    t.integer :value
  end
end

class Setting < ActiveRecord::Base
  def self.alias(klass, name, selector)
    klass.class_eval do
      define_method(name) do
        Setting.find_by(name: selector).value
      end
      private name
    end
  end
end

Setting.create! name: 'very_long_setting_name', value: 12

########################### First variant ###########################

class A
  Setting.alias self, :short_name, 'very_long_setting_name'

  def private_setting_short_name
    short_name
  end
end

A.new.private_setting_short_name # => 12

########################### Second variant ###########################

module Settings
  def setting(name, selector:)
    define_method(name) do
      Setting.find_by(name: selector).value
    end
    private name
  end
end

class B
  extend Settings

  setting :short_name, selector: 'very_long_setting_name'

  def private_setting_short_name
    short_name
  end
end

B.new.private_setting_short_name # => 12