Reduce validation messages

reduce_validator.rb

class ReduceValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return until record.errors.to_hash.has_key?(attribute)
    record.errors[attribute].slice!(-1) until record.errors[attribute].size <= 1
  end
end

reduce_validator_spec.rb

require 'rails_helper'
require 'rspec/active_model/mocks'

describe ReduceValidator do
  let(:reduce_validator) { ReduceValidator.new(attributes: {}) }

  let(:item) { mock_model("Item") }
  subject { item }

  before(:each) do
    item.errors.add(:name, "message one")
    item.errors.add(:name, "message two")
  end

  it { expect(subject).to have(2).error_on(:name) }

  it "should reduce error messages" do
    reduce_validator.validate_each(item, :name, '')
    expect(subject).to have(1).error_on(:name)
  end
end

validation.rb

validates :nickname,
  presence: true, 
  uniqueness: true,
  length: { minimum: 2, maximum: 20 },
  format: { with: Profile::Base::NICKNAME_REGEX },
  reduce: true