db.rake

namespace :db do
  desc 'Open a MongoDB console with connection parameters for the current Rails.env'
  task :console, [:session] => :environment do |t, args|
    session_name = args[:session] || :default

    config = Mongoid.sessions[session_name]
    username = config[:username]
    password = config[:password]
    database = config[:database]

    session = Mongoid::Sessions.with_name(session_name)
    host = session.cluster.with_primary {|node| node.resolved_address}

    args = []
    args << "--username=#{username}" if username
    args << "--password=#{password}" if password
    args << "--host=#{host}"
    args << database
    
    exec 'mongo', *args
  end
end

Brought up-to-date to support replica sets and Mongoid sessions. It will always connect to the primary automatically.

Usage:

rake db:console[session]