Question

How do you get access to the master node in Mongoid > 3.0 ?

In Mongoid < 3.0 you could use:

Mongoid::Config.master.eval('...')

The closest I can find in 3 seems to be:

klass.collection.database.command(eval: '...') #=> failed with error "not master"

Is there a better way to get access to master? Or a way to ensure the command is evaluated by the master node?

Answer 1

Mongoid 3.0 uses Moped and not the 10gen driver, so see Moped::Cluster#with_primary

http://rubydoc.info/github/mongoid/moped/master/Moped/Cluster:with_primary

For example:

User.collection.database.session.cluster.with_primary do
  p User.collection.database.command(eval: 'function() { return 3+3; }')
  p User.collection.database.command(ping: 1)
end

output:

{"retval"=>6.0, "ok"=>1.0}
{"ok"=>1.0}

Note that other possible solutions like group, aggregation, and mapreduce are recommended over eval.