code.rb

require "bcrypt"

module GoTipping
  class UserAuthenticator 
    def initialize(organisation, username, finder)
      @organisation = organisation
      @username = username
      @finder = finder
    end

    def authenticate(password)
      return false unless user

      if BCrypt::Password.new(user.password_digest) == password
        user
      else
        false
      end
    end

    private

    def user
      @_user ||= @finder.by_organisation_and_username(@organisation, @username)
    end
  end

  class UserFinder
    def by_organisation_and_username(organisation, username)
      organization.users.find_by(username: username)
    end
  end
end

spec.rb

require_relative "gistfile1"

RSpec.describe GoTipping::UserAuthenticator do

  subject { described_class.new(organisation, username, finder) }
  let(:username) { "foo" }
  let(:finder) { instance_double(GoTipping::UserFinder) }
  let(:organisation) { double("Organization") }
  let(:user) { double("User", password_digest: digest) }
  let(:digest) { "$2a$10$jYOHOwYW6PcYGUG6IafkV.WAqFY86sfkzjqOxxmycp7UozxXRr5ia" }

  before do
    expect(finder).to receive(:by_organisation_and_username).
      with(organisation, username).
      and_return(user)
  end

  context "user not found" do
    let(:user) { nil }
    it "is false" do
      expect(subject.authenticate("foo")).to eq(false)
    end
  end

  context "password wrong" do
    it "is false" do
      expect(subject.authenticate("wrong")).to eq(false)
    end
  end

  context "password correct" do
    it "returns the user" do
      expect(subject.authenticate("right")).to eq(user)
    end
  end

end