Say you have an app not dissimilar to StackOverflow where users vote on 
posts. Say an Up Vote causes the voter to receive one reputation point. You 
might see something like this in the vote_up model:

class UpVote < ActiveRecord::Base
  belongs_to :user
  after_create :increase_user_reputation

  private

  def increase_user_reputation
    user = self.user
    user.reputation == user.reputation + 1
  end
end

There is one issue with this code: The value 1 is hard coded. Where does 
such a value belong? Another issue is that some UpVote should have no 
knowledge of user.reputation. We solve this by changing the callback:

  def increase_user_reputation
    self.user.modify_reputation(1)
  end

Then we add an instance method to User:

  def modify_reputation(reputation)
    self.reputation = self.reputation + reputation
    self.save!
  end

Questions...

1. Who's responsible for calling save! ? The *increase_user_reputation*inside 
the UpVote model or the 
*modify_reputation* method inside the user model?
2. The +1 one to reputation doesn't seem like a good idea to hard code. 
Where does Rails keep such configuration settings?
3. Does the code look reasonable? Are there any shortcuts?

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