Keep code struture in models consistent

Following Rails conventions, life is happy. It can be happier if we create conventions for codes in models, especially when there's multiple programmers working on the same model.

One example: (From top to bottom)

1. associations
2. scopes
3. class methods
4. validates
5. callbacks
6. instance methods

Code example:

class Article < ActiveRecord::Base
  has_many :comments
  belongs_to :author

  default_scope order("id desc")
  scope :published, where(:published => true)
  scope :created_after, lambda{|time| ["created_at >= ?", time]}

  class << self
    def batch_create(data)
      # ...
    end
  end

  validates :title, :presence => true

  before_create :init_score
  def init_score
    self.score = 10
  end

  def any_instance_method
    # ...
  end

  begin "score related functions" # Group functions by begin .. end
    def add_score(score)
      # ...
    end
  end

end