September 21, 2010

Problem: mongod doesn’t start and the tail of /var/log/mongodb/mongod.log says:

Mon Sep 20 12:00:16 exception in initAndListen std::exception: old lock file, terminating

Solution: you somehow did not shut down the machine properly the last time. Remove the old lock file:

sudo rm /var/lib/mongodb/mongod.lock

And if you’re on Ubuntu, you can use the new Upstart syntax to start the process via the init.d script that the package from 10gen PPA provides:

sudo start|stop mongodb

Problem: given the following Mongoid document:

class Post
  include Mongoid::Document
  field :tags, :type => Array
end

you’d like to find all posts that contain a certain tag.

Solution: use any_in criteria. It may not be obvious from the docs, but it does apply to fields of type Array too:

Post.any_in(:tags => [t])