message_starred.rb

class Message
  class Starred
    attr_accessor :user, :message

    InvalidUserError = Class.new(StandardError)

    def initialize(user, message)
      self.user    = user
      self.message = message

      raise ArgumentError.new('User & message must be present') unless args_present?
    end

    def save
      raise InvalidUserError.new("User can't star own message") if user_owns_message?

      message.starred_by_users << user
    end

    def user_owns_message?
      message.user_id == user.id
    end

    private

    def args_present?
      !user.nil? && !message.nil?
    end
  end
end

message_starred_spec.rb

require 'spec_helper'

describe Message::Starred do
  let(:user_with_message) { double(:user, id: 1) }
  let(:user)              { double(:user, id: 2) }

  let(:starred_by_users) { double(:collection) }
  let(:message) { double(:message, user_id: 1, starred_by_users: starred_by_users) }

  context 'when args are missing' do
    it 'raises argument error for no user' do
      expect {
        described_class.new(nil, message)
      }.to raise_error(ArgumentError)
    end

    it 'raises argument error for no message' do
      expect {
        described_class.new(user_with_message, nil)
      }.to raise_error(ArgumentError)
    end
  end

  describe '#save' do
    let(:object) { described_class.new(user_context, message) }

    context 'when user is an owner of message' do
      let(:user_context) { user_with_message }

      it 'raises error' do
        expect {
          object.save
        }.to raise_error(described_class::InvalidUserError)
      end
    end

    context 'when user is not an owner of message' do
      let(:user_context) { user }

      it 'stars the message by user' do
        expect(starred_by_users).to receive(:<<).with(user)

        object.save
      end
    end
  end
end