gistfile1.rb

# This hack is designed to prevent a thread from implicitly checking out an AR connection by
# just accessing ActiveRecord::Base.connection. The point of doing this is to ensure that threads
# don't tie up connections that they are not using, by making sure we're explicit about where we
# need to use an AR connection.
#
# See also http://tenderlovemaking.com/2011/10/20/connection-management-in-activerecord.html

ActiveSupport.on_load(:active_record) do
  ActiveRecord::ConnectionAdapters::ConnectionPool # ensure loaded

  if ActiveRecord::VERSION::STRING !~ /^4.0/
    raise "This hack was written for Active Record 4.0. It needs to be checked " \
          "in order to use it with #{ActiveRecord::VERSION::STRING}."
  end

  module ActiveRecord
    class ImplicitConnectionDisabled < ConnectionNotEstablished
    end

    module DisableImplicitConnection
      def connection
        if conn = @reserved_connections[current_connection_id]
          conn
        else
          if Thread.current[:allow_implicit_connection]
            reserve_connection
          else
            raise ImplicitConnectionDisabled
          end
        end
      end

      def reserve_connection
        synchronize do
          @reserved_connections[current_connection_id] ||= checkout
        end
      end

      def allow_implicit_connection
        Thread.current[:allow_implicit_connection]
      end

      def allow_implicit_connection=(val)
        Thread.current[:allow_implicit_connection] = val
      end

      def with_connection
        prev, self.allow_implicit_connection = self.allow_implicit_connection, true
        super { |c| yield c }
      ensure
        self.allow_implicit_connection = prev
      end
    end

    module ReserveRackConnection
      def call(env)
        ActiveRecord::Base.connection_pool.allow_implicit_connection = true
        super
      end
    end

    module ConnectionAdapters
      class ConnectionPool
        prepend DisableImplicitConnection
      end

      class ConnectionManagement
        prepend ReserveRackConnection
      end
    end
  end

  Thread.main[:allow_implicit_connection] = true
end