test.rb

class Redis
  attr_accessor :server

end

class RedisNamespace
  attr_accessor :redis
  def initialize(namespace, options = {})
   self.redis = options[:redis] || :internal
  end

  def test
    @redis
  end
end

redis = Redis.new
redis.server = :external
activity = RedisNamespace.new(:activity, :redis => redis.server)
puts "redis.server = #{redis.server}"
puts "activity.redis = #{activity.test}"

# now change the server - similar to swapping in the specs
redis.server = :changed
puts "redis.server = #{redis.server}"
puts "activity.redis = #{activity.test}"

And the output

-> % ruby lib/test.rb
redis.server = external
activity.redis = external
redis.server = changed
activity.redis = external

@adamcooper shouldn't you be setting up activity like this:

RedisNamespace.new(:activity, :redis => redis)