_form.html.haml

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.5/jquery.min.js"></script>
= simple_form_for(@account) do |f|
  = f.error_notification

  = f.input :type, collection:['iban', 'rib'], include_blank: false

  #iban
    .inputs
      = f.input :iban, :include_blank => false
      = f.input :bic, :include_blank => false
      = f.input :address
    .action_views
      = f.button :submit

  #rib{ style: 'display: none;' }
    .inputs
      = f.input :bank, :include_blank => false
      = f.input :office, :include_blank => false
      = f.input :account, :include_blank => false
      = f.input :key, :include_blank => false
      = f.input :address
    .action_views
      = f.button :submit

:javascript
  $("#account_type").change(function() {
  var selection = $("#account_type").val();
  if (selection == 'rib')
  {
  $("#iban").hide();
  $("#rib").show();
  }
  if (selection == 'iban')
  {
  $("#rib").hide();
  $("#iban").show();
  }
  });

account.rb

class Account < ActiveRecord::Base
  belongs_to :user
  before_validation :iban_xor_rib
  self.inheritance_column = nil

  def rib?(rib)
    bank, office, account, key = rib.match(/^(\d{5})(\d{5})([[:alnum:]]{11})(\d{2})$/).captures
    account                    = account.each_char.inject('') do |sum, char|
      char = char.to_i 36
      sum  += ((char + 2 ** ((char - 10)/9)).to_i % 10).to_s
    end
    result                     = bank + office + account + key
    result.size >= 21 && result.to_i % 97 == 0
  rescue
    false
  end

  def iban_xor_rib
    if type == 'iban'
      validates_format_of :iban, allow_blank: false, with: /\A[A-Z]{2}/i, message: 'Country code is missing from the IBAN code'
    end
    if type == 'rib'
      rib = bank + office + account + key
      #   rib = rib.delete(' ')
      validates_format_of :account , allow_blank: false, with: rib?(rib), message: 'RIB is not valid'
    end
  end


end

accounts_controller.rb

class AccountsController < ApplicationController
  before_action :set_account, only: [:show, :edit, :update, :destroy]

  # GET /accounts
  # GET /accounts.json
  def index
    @accounts = current_user.accounts.all
  end

  # GET /accounts/1
  # GET /accounts/1.json
  def show
    authorize! :read, @account
  end

  # GET /accounts/new
  def new
    @account = current_user.accounts.new
    authorize! :create, @account
  end

  # GET /accounts/1/edit
  def edit
    authorize! :update, @account
  end

  # POST /accounts
  # POST /accounts.json
  def create
    @account = current_user.accounts.new(account_params)
    authorize! :create, @account

    respond_to do |format|
      if @account.save
        format.html { redirect_to @account, notice: 'Account was successfully created.' }
        format.json { render action: 'show', status: :created, location: @account }
      else
        format.html { render action: 'new' }
        format.json { render json: @account.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /accounts/1
  # PATCH/PUT /accounts/1.json
  def update
    authorize! :update, @account

    respond_to do |format|
      if @account.update(account_params)
        format.html { redirect_to @account, notice: 'Account was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @account.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /accounts/1
  # DELETE /accounts/1.json
  def destroy
    authorize! :destroy, @account

    @account.destroy
    respond_to do |format|
      format.html { redirect_to accounts_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_account
      @account = current_user.accounts.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def account_params
      params.require(:account).permit(:type, :iban, :bic, :bank, :office, :account, :key, :address)
    end
end