Sinatra using ActiveRecord's Query Caching ability across multiple Databases : http://proccli.com/active-record-query-caching-sinatra-multiple-databases

config.ru

require 'sinatra_query_caching_demo'
run Demo

sinatra_query_caching_demo.rb

require 'rubygems'
require 'sinatra/base'
require 'active_record'
require 'logger'

# override some instance variables and _force_ query caching to be enabled on databases other than AR::Base's connection
module ARQueryCachingHack
  def self.included(klass)
    # make sure you call this _AFTER_ a connection has been established, otherwise the connection
    # will override these. it is kind of a pain, so the best bet is to include this in the initialization
    def klass.enable_query_caching
      self.connection.instance_variable_set(:@query_cache_enabled, true)
      self.connection.instance_variable_set(:@query_cache, {})
    end
    def klass.disable_query_caching
      self.connection.instance_variable_set(:@query_cache_enabled, false)
    end
  end
end

ActiveRecord::Base.configurations = {
  # this is a garbage connection so we can make sure AR::Base is on a different connection. If we are sharing
  # the connection it kind of renders this moot since AR::Base is always query cached with the AR::QueryCache
  # middleware
  'test' => {
    'adapter' => 'mysql',
    'host' => 'localhost',
    'username' => 'root',
    'password' => '',
    'database' => 'testing' # change this to some other DB besides +mysql+
  },
  # our separate target database. if we are on an app that uses multiple DB's then this would be the +other+ db, not
  # AR::Base
  'mysql' => {
    'adapter' => 'mysql',
    'host' => 'localhost',
    'username' => 'root',
    'password' => '',
    'database' => 'mysql'
  }
}

# AR::Base needs a connection before AR::QueryCache can take effect
ActiveRecord::Base.establish_connection 'test'
ActiveRecord::Base.logger = Logger.new($stdout)

# just to keep things separate, tuck all external models in their own module
module MySQL
  # this would be a common base for all models that need a different DB, since all AR:Base models will just
  # inherit from AR::Base
  class Base < ActiveRecord::Base
    self.abstract_class = true
    establish_connection 'mysql'
  end
  # some model in a different database
  class User < Base
    include ARQueryCachingHack
    set_table_name :user
  end
  # enable query caching on each model you want
  def self.query_cache_models
    [User].each &:enable_query_caching 
  end
end

class Demo < Sinatra::Base
  use ActiveRecord::QueryCache
  
  before do
    # enable query caching by default. this probably isn't where you _want_ to put this, an initializer is probably best
    MySQL.query_cache_models
    ActiveRecord::Base.logger.info '-- enabling up per-model query caching --'
  end

  get '/' do
    # with query caching
    10.times{ MySQL::User.first }
    # => CACHE (0.0ms)   SELECT * FROM `user` LIMIT 1
    
    # without query caching
    ActiveRecord::Base.logger.info '-- disabling up per-model query caching --'
    MySQL::User.disable_query_caching
    10.times{ MySQL::User.first }
    # => MySQL::User Load (0.3ms)   SELECT * FROM `user` LIMIT 1
    
    "Hack0rific!"
  end
end