https://www.facebook.com/groups/rubykr/permalink/741366539209376/?stream_ref=2

1.rb

class A < ActiveRecord::Base
  validates :phoneNum1, numericality: { only_integer: true }
  validates :phoneNum2, numericality: { only_integer: true }
  validates :phoneNum3, numericality: { only_integer: true }
  
  def phoneNum1
    phoneNum && phoneNum.split("-")[0]
  end
  def phoneNum2
    phoneNum && phoneNum.split("-")[1]
  end
  def phoneNum3
    phoneNum && phoneNum.split("-")[2]
  end
  def phoneNum1=(phoneNum1)
    @phoneNum1 = phoneNum1
    set_phoneNum
  end
  def phoneNum2=(phoneNum3)
    @phoneNum2 = phoneNum3
    set_phoneNum
  end
  def phoneNum3=(phoneNum3)
    @phoneNum3 = phoneNum3
    set_phoneNum
  end
  private
  def set_phoneNum
    if @phoneNum1 && @phoneNum2 && @phoneNum3
      self.phoneNum = "#{@phoneNum1}-#{@phoneNum2}-#{@phoneNum3}"
    end
  end
end

2.rb

class A < ActiveRecord::Base
  validates :phoneNum, ...
end


class AController < ApplicationController
  def create
    A.create(a_params)
  end

  private
  def a_params
    params[:a][:phoneNum] = "#{params[:a][:phoneNum1]}-#{params[:a][:phoneNum2]}-#{params[:a][:phoneNum3]}"
    params[:a].permit(...)
  end
end