#Theirs:

class UsersController < ApplicationController
  def follow
    @user = User.find(params[:id])
    if current_user.follow(@user)
      redirect_to root_url
    else
      redirect_to @user
    end
  end
end

#Mine:

class UsersController < ApplicationController
  def follow
    @user = User.find(params[:id])
    current_user.follow(@user) ? redirect_to root_url : redirect_to @user
  end
end

#Theirs:

class User < ActiveRecord::Base
  has_many :followings
  def follow(user)
    unless followings.where(:followed_user_id => user.id).present?
      followings.create(:followed_user => user)
    else
      false
    end
  end
end

#Mine:

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