mongo_mapper_enable_identity_map.rb

class Foo
  plugin MongoMapper::Plugins::IdentityMap
end
 
Or you can add to all documents like this:
 
module IdentityMapAddition
  def self.included(model)
    model.plugin MongoMapper::Plugins::IdentityMap
  end
end
 
MongoMapper::Document.append_inclusions(IdentityMapAddition)

mongo_mapper_identity_map_middleware.rb

class PerRequestCache
  def initialize(app)
    @app = app
  end
 
  def call(env)
    MongoMapper::Plugins::IdentityMap.clear
    @app.call(env)
  ensure 
    MongoMapper::Plugins::IdentityMap.clear
  end
end

config.middleware.use 'PerRequestCache'