errors.rb

require 'forwardable'

module Api
  class Errors
    extend Forwardable

    def_delegators :errors, :<<, :to_ary, :empty?, :count

    def initialize(errors=[])
      @errors = Array(errors)
    end

    def +(other)
      self.class.new(errors + other.to_errors.to_ary)
    end

    def as_json
      { errors: errors }
    end

    private

    attr_reader :errors
  end
end

errors_spec

require 'spec_helper'

describe Api::Errors do
  subject(:errors){ described_class.new }

  describe '+' do
    it 'returns errors when errors are added together' do
      expect(errors + errors).to be_a(described_class)
    end

    it 'returns errors when an array is added to it' do
      expect(errors + []).to be_a(described_class)
    end
  end

  describe '#as_json' do
    it 'wraps the output in a top-level "errors" node' do
      expect(errors.as_json).to include(:errors)
    end
  end
end