how to solve 'Mysql2::Error: This connection is still waiting for a result' error with mysql2 and activerecord - eventmachine

Question

Not duplicate of this question with the same title
I am using activerecord with mysql2 and I am designing to handle 10 queries to the same activerecord model/class at a time. Please note I am using strict activerecord and not using mysql queries directly.
I get the calls in Sinatra and then use activerecord to get the data from the DB.
I don't wan't the calls to be blocking so therefore I used mysql2 and I do NOT want to use em-synchrony.
But now I get the following "Mysql2::Error: This connection is still waiting for a result, try again once you have the result:" on subsequent simultanous calls.
I am not establishing connection with pool=10
my class
class User < ActiveRecord::Base
and my code to call
user.find(:all, :conditions => ["id=?", userid])
The mysql2 doc says "To use the ActiveRecord driver (with or without rails), all you should need to do is have this gem installed and set the adapter in your database.yml to "mysql2". That was easy right? :)"
And that is exactly what I have done when I moved from mysql to mysql2.
Why am I getting this error.

Answer 1

Here is a fully working example:
require 'rubygems'
gem 'activerecord', '~> 3.1.0'
gem 'sinatra', '~> 1.3.1'
gem 'mysql2', '~> 0.3.11'
require 'active_record'
require 'sinatra/base'
require 'mysql2'
# thin use the eventmachine thread pool
# you should have at least one connection per thread
# or you can expect errors
EM::threadpool_size = 10
# connect to the database
ActiveRecord::Base.establish_connection(
:adapter => "mysql2",
:database => "test",
:username => "root",
:encoding => 'utf8',
# number of connections openened to the database
:pool => 10
)
class App < Sinatra::Base
get '/db' do
ActiveRecord::Base.connection.execute("SELECT SLEEP(1)")
end
end
run App
to run it save the file as "config.ru" and run it with thin in threaded mode:
thin start -e production --threaded
You can use ab to check that everything is working, I used a tool called siege:
siege -c 10 -r 1 http://localhost:3000/db

Answer 2

You should use a ConnectionPool... Somehow you have 2 connections in a race condition.
I dont use Sinatra, I use Rails, but I had the same problem and solved it like that:
# class ActiveRecord::Base
# mattr_accessor :shared_connection
# ##shared_connection = nil
#
# def self.connection
# ##shared_connection || retrieve_connection
# end
# end
#
# ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection
class ActiveRecord::Base
mattr_accessor :shared_connection
##shared_connection = nil
def self.connection
##shared_connection || ConnectionPool::Wrapper.new(:size => 1) { retrieve_connection }
end
end
ActiveRecord::Base.shared_connection = ActiveRecord::Base.connection

Answer 3

You have to use a connection pool. mysql2 allows the queries to be async, but you can still only send one query at a time to MySQL through one connection. If you send multiple queries through one connection you get the waiting for results message.
Use a connection pool and you should be fine.