_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
  validate :iban_xor_rib
  self.inheritance_column = nil

  private

  def iban_xor_rib
    if type == 'rib'
      errors.add type, :mandatory and return if [bank, office, account, key].any? &:blank?
      rib = "#{bank}#{office}#{account}#{key}"
      iban = "#{rib}FR00".each_char.inject('') do |sum, char|
        sum += char.to_i(36).to_s
      end.to_i
      iban = "FR%.2d%s" % [98 - (iban % 97), rib]
    end

    errors.add type, :mandatory and return if iban.blank?
    errors.add type, :country and return unless iban.to_s =~ /^[A-Z]{2}/i
    tmp = iban.gsub(/[A-Z]/) { |p| (p.respond_to?(:ord) ? p.ord : p[0]) - 55 }
    errors.add type, :format unless (tmp[6..tmp.length-1].to_s+tmp[0..5].to_s).to_i % 97 == 1
    errors.add type, :country unless iban =~ /\A[A-Z]{2}/i
  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