payer_decorator.rb

# encoding: utf-8
class PayerDecorator < Draper::Decorator
  delegate_all
  decorates :payer

  def inn_unique?(inn)
    Payer.where { (major_requisite.eq 'INN') & (major_requisite_value.eq inn) & (id.not_eq id) }.exists?
  end
end

payer_decorator_spec.rb

# encoding: utf-8
require 'spec_helper'

describe PayerDecorator do
  describe '#inn_unique?' do
    subject { payer.decorate.inn_unique?(inn) }
    let(:inn) { '1231231230' }
    let(:payer) { create :payer, major_requisite: 'INN', major_requisite_value: inn }
    before { create :payer, major_requisite: 'INN', major_requisite_value: '1231230000' }

    context 'when exists only one payer with such inn' do
      it { expect(subject).to be_true }
    end

    context 'when exists two payers with the same inn' do
      before { create :payer, major_requisite: 'INN', major_requisite_value: inn }
      it { expect(subject).to be_false }
    end
  end
end