Rails: Update model attribute without invoking callbacks

I have a User model that has a :credits attribute. I want a simple button that will add 5 to the user's credits, through a route called "add" so that /users/3/add would add 5 to the credits of user id = 3.

def add
    @user = User.find(params[:id])
    @user.credits += 5
    redirect_to root_path
end

That is the relevant part of my controller. The problem is, I dont want to call @user.save because I have a before_save callback that re-encrypts the user's password based on the current UTC time. I just want to simply add 5 to the attribute and avoid the callback, I never thought such a simple thing could be so hard.

EDIT:

I changed the callback to :before_create, here is my new controller code (relevant part):

  def add
    @user = User.find(params[:id])
    @user.add_credits(5)
    @user.save
    flash[:success] = "Credits added!"
    redirect_to root_path
  end

and here is my code in the model:

 def add_credits(num)
    self.credits = num
 end

EDIT 2:

Ok it was a validation problem that made the changes in "EDIT" not work, but I'd still love an answer to the original question of updating without callbacks!

Rails 3.1 introduced update_column, which is the same as update_attribute, but without triggering validations or callbacks:

http://apidock.com/rails/ActiveRecord/Persistence/update_column

As a general answer, in Rails 4 this is a simple way to update attributes without triggering callbacks:

@user.update_column :credits, 5

If you need to update multiple attributes without triggering callbacks:

@user.update_columns credits: 5, bankrupt: false  

There are other options here in the Rails Guides if you prefer, but I found this way to be the easiest.