I decided to use Moped for this task.
require 'moped'
def connect_mongo(config)
session = Moped::Session.new(["#{config['host']}:#{config['port']}"])
session.use config['database']
session.login config['username'],config['password']
session
end
And the code of saving user to another DB:
mongo = connect_mongo(config1)
mongo_staging = connect_mongo(config2)
users = mongo[:users].find.limit(limit)
users.each do |user|
# some code here ...
mongo_staging[:users].insert(user)
end