db_sharding.rb

require 'sequel'
require 'delegate'

module DB
  extend self

  class NoCurrentShardError < StandardError; end

  class Shard < ::Delegator
    Spec = Struct.new(:host, :database) do
      def connection_string
        "mysql2://root@#{host}:3306/#{database}"
      end
    end

    def spec=(spec)
      Thread.current[:db_shard_spec] = spec
      Thread.current[:db_shard] = nil
    end

    def spec
      Thread.current[:db_shard_spec]
    end

    def __getobj__
      Thread.current[:db_shard] ||= new_connection
    end

    # Ensure #send is proxied to the Sequel DB instance
    undef send

  private

    def initialize
      super(nil)
    end

    def new_connection
      spec = Thread.current[:db_shard_spec]
      raise NoCurrentShardError unless spec
      ::Sequel.connect(spec.connection_string)
    end

    def __setobj__(obj)
      # no-op
    end
  end

  def shard
    @shard ||= Shard.new
  end

  def use_shard(*args)
    orig_shard_spec = shard.spec
    shard.spec = Shard::Spec.new(*args)
    begin
      yield
    ensure
      shard.spec = orig_shard_spec
    end
  end
end

# use the current sharded DB for our models by default
Sequel::Model.db = DB.shard

usage.rb

require 'sinatra'

class Widget < Sequel::Model
end

get '/:user_id/widgets' do
  host, database = ShardMaster.host_and_db_for(params[:user_id])
  DB.use_shard(host, database) do
    Widget.all.to_json
  end
end