gistfile1.rb

#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 

In your example the ternary is evaluated before the call to current_user.follow

current_user.follow @user ? redirect_to root_url : redirect_to @user

Hopefully this will demonstrate part of the issue:

$ irb --simple-prompt
>> def foo(bar)
>>   puts bar
>>   bar == 'baz'
>> end                                                                                                                                                                                                                                        
=> nil
>> foo 'baz' ? 'left' : 'right'
(irb):10: warning: string literal in condition
left
=> false
>> foo('baz') ? 'left' : 'right'
baz
=> "left"

If you notice in the first call to foo (foo 'baz' ? 'left' : 'right'), the value of bar is not baz but left.

You will need to put parens all over the place for that ternary to work how you want:

current_user.follow(@user) ? redirect_to(root_url) : redirect_to(@user)