Form entry uniqueness validation and test

my_form_object.rb

# /app/form_objects/my_form_object.rb
class MyFormEntry < FormEntry
  attribute :reference, String
  
  validate :unique_reference

  def unique_reference
    errors.add(:reference, "must be unique") if \
      MyObject.find_by(reference: self.reference)
  end
end

my_form_object_spec.rb

# /spec/form_objects/my_form_object_spec.rb
require 'rails_helper'

RSpec.describe MyFormObject, type: :model do
  describe '#valid?' do
    let(:entry) { MyFormObject.new(reference: Faker::Number.number(6)) }
    subject { entry }

    describe 'custom validators are invoked' do
      before do
        allow(entry).to receive(:unique_reference)

        entry.valid?
      end

      it { is_expected.to have_received(:unique_reference) }
    end

    context 'when the reference exists' do
      before do
        allow(MyObject)
          .to receive(:find_by)
          .with(reference: entry.reference)
          .and_return(double(:my_object))

        entry.valid?
      end

      its(:valid?) { is_expected.to be false }
      it 'should add an error to reference' do
        expect(entry.errors.messages[:reference]).to be_present
      end
    end

    context 'when the reference does not exist' do
      before do
        allow(MyObject)
          .to receive(:find_by)
          .with(reference: entry.reference)
          .and_return(nil)

        entry.valid?
      end

      its(:valid?) { is_expected.to be true }
      it 'should not add an error to reference' do
        expect(entry.errors[:reference]).to be_empty
      end
    end
  end
end