Move Model Logic into the Model

Bad Smell

class PostsController < ApplicationController
  def publish
    @post = Post.find(params[:id])
    @post.update_attribute(:is_published, true)
    @post.approved_by = current_user
    if @post.created_at > Time.now - 7.days
      @post.popular = 100
    else
      @post.popular = 0
    end

    redirect_to post_url(@post)
  end
end

In this example, PostsController wants to publish a post, first it finds a post, set post's attribute is_published and approved, and assigns the popular attribute according to the value of created_at, PostsController knows too much about the logic, it is not controller's responsibility, it should be handled by model.

Refactor

class Post < ActiveRecord::Base
  def publish
    self.is_published = true
    self.approved_by = current_user
    if self.created_at > Time.now - 7.days
      self.popular = 100
    else
      self.popular = 0
    end
  end
end

class PostsController < ApplicationController
  def publish
    @post = Post.find(params[:id])
    @post.publish

    redirect_to post_url(@post)
  end
end

Now we move the publish logic from controller into the model, create a publish method for Post model, then we just call the publish method in controller. It looks beautiful.