Refactor the code to delegate logic to the model

good_skinny_controller.rb

# Model
class User < ActiveRecord::Base
  has_many :followings

  def follow(user)
    self.followings.create(:followed_user => user) unless self.followings.where(:followed_user_id => user.id).present?
  end
end

# Controller
class UsersController < ApplicationController
  def follow
    @user = User.find(params[:id])

    if current_user.follow(@user).nil?
      redirect_to @user
    else
      redirect_to root_url
    end
  end
end

poor_skinny_controller.rb

# Model
class User < ActiveRecord::Base
  has_many :followings
end

# Controller
class UsersController < ApplicationController
  def follow
    @user = User.find(params[:id])

    if current_user.followings.where(:followed_user_id => @user.id).present?
      redirect_to @user
    else
      current_user.followings.create(:followed_user => @user)
      redirect_to root_url
    end
  end
end