I have problem when updating non existing field of Game model

My Game model is -

class Game
include Mongoid::Document
field :name
end

When i am trying to update field number_of_players which doesn't exist in model using update_attribute

Game.last.update_attribute(:number_of_players, 11)

It's creating field in mongodb and my log is

MOPED: 127.0.0.1:27017 QUERY database=game_development collection=games selector= {"$query"=>{}, "$orderby"=>{:_id=>-1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil runtime: 0.7107ms
MOPED: 127.0.0.1:27017 UPDATE database=game_development collection=games selector=

{"_id"=>BSON::ObjectId('547df1f17368773b70010000')}

update={"$set"=>{"number_of_players"=>11}} flags=[]
COMMAND database=game_development command={:getlasterror=>1, :w=>1} runtime: 0.6224ms

Here is the record from mongo console

> db.games.find(

{name: 'cricket'}

)

{ "_id" : ObjectId("547df1f17368773b70010000"), "name" : "cricket", "number_of_players" : 11 }

Now when i am trying to do same thing using update attributes

Game.last.update_attributes(number_of_players: 11)

It's raising error

Mongoid::Errors::UnknownAttribute:
Problem:
Attempted to set a value for 'number_of_players' which is not allowed on the model Game.
Summary:
Without including Mongoid::Attributes::Dynamic in your model and the attribute does not already exist in the attributes hash, attempting to call Game#number_of_players= for it is not allowed. This is also triggered by passing the attribute to any method that accepts an attributes hash, and is raised instead of getting a NoMethodError.
Resolution:
You can include Mongoid::Attributes::Dynamic if you expect to be writing values for undefined fields often.
from /home/shweta/.rvm/gems/ruby 2.1.0/gems/mongoid4.0.0/lib/mongoid/attributes/processing.rb:96:in `process_attribute'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid 4.0.0/lib/mongoid/attributes/processing.rb:25:in `block in process_attributes'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid-4.0.0/lib/mongoid/attributes/processing.rb:23:in `each_pair'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid-4.0.0/lib/mongoid/attributes/processing.rb:23:in `process_attributes'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid-4.0.0/lib/mongoid/attributes.rb:203:in `block in assign_attributes'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid-4.0.0/lib/mongoid/threaded/lifecycle.rb:26:in `_assigning'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid-4.0.0/lib/mongoid/attributes.rb:202:in `assign_attributes'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/mongoid-4.0.0/lib/mongoid/persistable/updatable.rb:51:in `update'
from (irb):31
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/railties-4.1.1/lib/rails/commands/console.rb:90:in `start'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/railties-4.1.1/lib/rails/commands/console.rb:9:in `start'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/railties-4.1.1/lib/rails/commands/commands_tasks.rb:69:in `console'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/railties-4.1.1/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
from /home/shweta/.rvm/gems/ruby-2.1.0/gems/railties-4.1.1/lib/rails/commands.rb:17:in `<top (required)>'
from bin/rails:4:in `require'
from bin/rails:4:in `<main>'

and log is

MOPED: 127.0.0.1:27017 COMMAND database=admin command={:ismaster=>1} runtime: 0.6409ms
MOPED: 127.0.0.1:27017 QUERY database=game_development collection=games selector={"$query"=>{}, "$orderby"=>{:_id=>-1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil runtime: 0.6466ms

is this a bug or i am missing something?