class Character
  include Mongoid::Document
  embeds_one :stats, :class_name => 'Stats'
end

class Stats
  include Mongoid::Document
  field :directions,  :type => Array
  embedded_in :character
end

1.9.3p0 :010 > cc = Character.first.stats
 => #<Stats _id: 4f071ee09759f7826800001d, _type: nil, directions: nil> 
1.9.3p0 :011 > cc.directions = [2, 3, 6, 2] 
 => [2, 3, 6, 2] 
1.9.3p0 :012 > cc.save
 => true 
1.9.3p0 :013 > cc
 => #<Stats _id: 4f071ee09759f7826800001d, _type: nil, directions: [2, 3, 6, 2]> 
1.9.3p0 :014 > cc = Character.first.stats
 => #<Stats _id: 4f071ee09759f7826800001d, _type: nil, directions: nil> 

1.9.3p0 :001 > cc = Character.first.stats
 => #<Stats _id: 4f071ee09759f7826800001d, _type: nil, directions: nil> 
1.9.3p0 :002 > c.directions = [4, 0, 7, 1]
 => [4, 0, 7, 1] 
1.9.3p0 :003 > cc.save
 => true 
1.9.3p0 :004 > cc
 => #<Stats _id: 4f071ee09759f7826800001d, _type: nil, directions: [4, 0, 7, 1]> 
1.9.3p0 :005 > cc = Character.first.stats
 => #<Stats _id: 4f071ee09759f7826800001d, _type: nil, directions: [4, 0, 7, 1]>