mongoid query caching - ruby

Question

Rails' ActiveRecord has a feature called Query Caching (ActiveRecord::QueryCache) which saves the result of SQL query for the life-span of a request. While I'm not very familiar with the internals of the implementation, I think that it saves the query results somewhere in the Rack env, which is discarded in the end of the request.
The Mongoid, unfortunately, doesn't currently provide such feature, and this is exacerbated by the fact, that some queries occur implicitly (references).
I'm considering to implement this feature, and I'm curious, where and how Mongoid (or, perhaps, mongo driver?) should be hooked in order to implement this.

Answer 1

Mongoid has caching, described under http://mongoid.org/en/mongoid/docs/extras.html
Also MongoDB itself has caching ability: http://www.mongodb.org/display/DOCS/Caching
The mongoid caching extra knows 2 different cases: Caching of all queries of a model or caching of a query.
Mongoid caching seems to work slightly different: it looks like mongoid delegates caching to mongodb. (In the sources of mongoid I only can find option settings for caching but no cache module.)
Finally would say, there is no real difference in the caching in general -- in memory is in fact in memory! No matter if it's in the app or in the database.
I don't prefer to implement an extra caching algorithm, because this seems to be redundant and a RAM killer.
BTW: If your really want to cache results in-app you could try Rails.cache or another cache gem as a workaround.

Answer 2

The other answer is obviously wrong. Not only mongoid or mongo driver doesn't cache the query, even if mongo would - it still might be on other machine across the network.
My solution was to wrap the receive_message in Mongo::Connection.
Pros: one definite place
Cons: deserialization still takes place
require 'mongo'
module Mongo
class Connection
module QueryCache
extend ActiveSupport::Concern
module InstanceMethods
# Enable the selector cache within the block.
def cache
#query_cache ||= {}
old, #query_cache_enabled = #query_cache_enabled, true
yield
ensure
clear_query_cache
#query_cache_enabled = old
end
# Disable the selector cache within the block.
def uncached
old, #query_cache_enabled = #query_cache_enabled, false
yield
ensure
#query_cache_enabled = old
end
def clear_query_cache
#query_cache.clear
end
def cache_receive_message(operation, message)
#query_cache[operation] ||= {}
key = message.to_s.hash
log = "[MONGO] CACHE %s"
if entry = #query_cache[operation][key]
Mongoid.logger.debug log % 'HIT'
entry
else
Mongoid.logger.debug log % 'MISS'
#query_cache[operation][key] = yield
end
end
def receive_message_with_cache(operation, message, log_message=nil, socket=nil, command=false)
if query_cache_enabled
cache_receive_message(operation, message) do
receive_message_without_cache(operation, message, log_message, socket, command)
end
else
receive_message_without_cache(operation, message, log_message, socket, command)
end
end
end # module InstanceMethods
included do
alias_method_chain :receive_message, :cache
attr_reader :query_cache, :query_cache_enabled
end
end # module QueryCache
end # class Connection
end
Mongo::Connection.send(:include, Mongo::Connection::QueryCache)

Answer 3

OK, Mongoid 4 supports QueryCache middleware.
Just add middleware in application.rb
config.middleware.use "Mongoid::QueryCache::Middleware"
And then profit:
MOPED: 127.0.0.1:27017 QUERY database=XXX collection=page_variants selector={"$query"=>{"_id"=>BSON::ObjectId('5564dabb6d61631e21d70000')}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil runtime: 0.4397ms
MOPED: 127.0.0.1:27017 QUERY database=XXX collection=page_variants selector={"$query"=>{"_id"=>BSON::ObjectId('5564dacf6d61631e21dc0000')}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil runtime: 0.4590ms
QUERY CACHE database=XXX collection=page_variants selector={"$query"=>{"_id"=>BSON::ObjectId('5564c9596d61631e21d30000')}, "$orderby"=>{:_id=>1}}
QUERY CACHE database=XXX collection=page_variants selector={"$query"=>{"_id"=>BSON::ObjectId('5564dabb6d61631e21d70000')}, "$orderby"=>{:_id=>1}}
Source:
Mongoid changelog
https://github.com/mongoid/mongoid/blob/master/CHANGELOG.md#new-features-2
3410 Mongoid now has a query cache that can be used as a middleware in Rack applications. (Arthur Neves)
For Rails:
config.middleware.use(Mongoid::QueryCache::Middleware)

Answer 4

Mongoid 4.0+ now has a QueryCaching module: http://www.rubydoc.info/github/mongoid/mongoid/Mongoid/QueryCache
You can use it on finds by wrapping your lookups like so:
QueryCache.cache { MyCollection.find("xyz") }