I'm trying to use inc on an embedded document, and it seems like the generated mongo command does not actually perform anything and the increment is lost.
To recreate the issue:
class User
include Mongoid::Document
embeds_many :game_stats
end
class GameStat
include Mongoid::Document
embedded_in :user
field :playcount, :type=>Integer, :default=>0
end

>> u=User.first
>> u.game_stats.create
>> u.save
>> u.game_stats.first.inc(:playcount, 1)
=> 1

Checking playcount again shows it did not in fact increment.

the mongo console shows the following command was sent:
mydbname.game_stats query:

{ _id: ObjectId('4d9b073e3149bc1124000002'), game_stats._id: ObjectId('4d9d7ce53149bc197c000002') }

update: { $inc:

{ playcount: 1 }

} 1ms

Seems like it should be using $inc:

{game_stats.0.playcount : 1}

, as it does with a regular save() call:
>> u.game_stats.first.playcount += 1
>> u.save
which translates into:
update mydbname.users query:

{ _id: ObjectId('4d9b073e3149bc1124000002') }

update: { $set:

{ game_stats.0.playcount:1 }

}