Specifies following options in "Mongo::Client".

• Connects to a mongos server. and
• Not specifies ":connect => :sharded". and
• Specifies "primary_preferred" to a read preference.
  Rerieves documents by using "find" method.

But "find" method runs well if following options are specified in "Mongo::Client".

• Specifies ":connect => :sharded". or
• Specifies "primary" to a read preference.

Ruby Code
=========
require 'mongo'
db = Mongo::Client.new(
['test-server:20001'],
{
:database => "test",
:user => "test_user",
:password => "test_pass",

1. :connect => :sharded, # find method runs well when ":connect => :sharded" is specified.
   :read => { :mode => :primary_preferred } # find method runs well when ":primary" is specified.
   }
   )
   cur = db["test"].find()
   col = nil; cur.each { |c| col = c; break }

   puts( col )

Result
======

{"$err"=>"Can't canonicalize query: BadValue unknown top level operator: $readPreference", "code"=>17287}