# MODEL
class User
  include Mongoid::Document

  field :username
  field :counter, :default => 0
  field :loves, :default => []

  index({ :username => 1 })
end

# DB SEED FILE
marc = User.new
marc.username = 'marc'
marc.counter = 5
marc.loves = ['a', 'b', 'c']
marc.save

# QUERY
User.where(:username => 'marc').
                          update_all(
                            "$inc" => {
                              :counter => -1,
                            },
                            "$pull" => {
                              :loves => 'b'
                            }
                          )

# RESULT IN DB
{
   "$inc": {
     "counter": -1
  },
   "$pull": {
     "loves": "b"
  },
   "_id": ObjectId("4fe08719bd31e0fbd7000001"),
   "counter": 5,
   "loves": {
     "0": "a",
     "1": "b",
     "2": "c"
  },
   "username": "marc"
}