gistfile1.rb

class MQ
  attr_reader :current_context
  def context(hash)
    @current_context ||= {}
    begin
      old_current_context = @current_context
      @current_context = @current_context.merge(hash)
      yield
    ensure
      @current_context = old_current_context
    end
  end

  def call(method,args,context=current_context)
    { "method" => method,
      "args" => args,
      "context" => context}
  end
end

class WithUserContext < MQ
  def call(method,args)
    super(method,args,current_context.merge("user_id" => current_user_id))
  end

  def current_user_id
    1
  end
end

p :MQ
mq = MQ.new
mq.context("foo" => 1) do
  p mq.call(:foo,[1])
  mq.context("foo" => 2,"bar" => 2) do
    p mq.call(:foo,[2])
  end
  p mq.call(:foo,[1])
end

# {"args"=>[1], "method"=>:foo, "context"=>{"foo"=>1}}
# {"args"=>[2], "method"=>:foo, "context"=>{"foo"=>2, "bar"=>2}}
# {"args"=>[1], "method"=>:foo, "context"=>{"foo"=>1}}

p :WithUserContext
mq = WithUserContext.new
mq.context("foo" => 1) do
  p mq.call :foo, []
end

# :WithUserContext
# {"args"=>[], "method"=>:foo, "context"=>{"user_id"=>1, "foo"=>1}}