Rspec: How to mock class method

Testing RSpec
1

Hi, I have two model: Message, User

message.rb

class Message < ActiveRecord::Base
  class << self
    def method_a(args)
      [1,2]
    end
  end
end

user.rb

class User < ActiveRecord::Base
  def method_user
    if Message.method_a('anythings')
      #... some code
    end
  end
end

I want to test method_user,but I don’t want to test Message#method_a
How I can mock Message with method_a?

Thanks!