When Memoization met DRY 💕

Let’s go on a journey into the world of memoization through the medium of ~~dance~~ Rails…

Once upon a time there was a controller with 3 typical methods:

class UserController
  def index
    clan = Clan.find(params[:clan_id])
    users = clan.users # a user belongs to a clan and a clan has many users

    render locals: {users: users, clan: clan}
  end

  def new
    clan = Clan.find(params[:clan_id])
    user = User.new
    render locals: {user: user, clan: clan}
  end

  def create
    clan = Clan.find(params[:clan_id])
    user = clan.users.new(user_params)

    if user.save
      redirect_to ...
    else
      render :new
    end

  end

  private

  def user_params
    params.require(:user).permit(:name, :power)
  end
end

And all was well and good until ‘DRY’ came along and said “No, not by the hairs on my chinny chin chin…”

In all 3 methods we have the exact same line of code clan = Clan.find(params[:clan_id])

Fear not, we can clean this up by using a before_action to do the exact same thing:

class UserController
  before_action :set_clan
  
  def index
    users = clan.users

    render locals: {users: users, clan: clan}
  end

  def new
    user = User.new
    render locals: {user: user, clan: clan}
  end

  def create
    user = clan.users.new(user_params)

    if user.save
      redirect_to ...
    else
      render :new
    end

  end

  private

  def user_params
    params.require(:user).permit(:name, :power)
  end
  
  def set_clan
    clan = Clan.find(params[:clan_id])
  end
end

“DRY” was happy with this and managed to calm down, until “Memoization” came along and said what if I can get rid of your before_action altogether 🤯

Check this out =>

class UserController  
  def index
    users = clan.users

    render locals: {users: users, clan: clan}
  end

  def new
    user = User.new
    render locals: {user: user, clan: clan}
  end

  def create
    user = clan.users.new(user_params)

    if user.save
      redirect_to ...
    else
      render :new
    end

  end

  private

  def user_params
    params.require(:user).permit(:name, :power)
  end
  
  def clan
    Clan.find(params[:clan_id])
  end
end

Notice what we did?

We removed the before_action and changed the set_clan method to be called clan.

This means that anytime the code calls clan it will not be calling a variable but rather the clan method.

But hold on a second, I spot a performance issue….

Take a look at the index method:

def index
  users = clan.users

  render locals: {users: users, clan: clan}
end

We call clan 2 times which means we will be hitting our database twice in order to retrieve the Clan record from the database.

Enter “Memoization”…

This’ll fix it 🪛

 def clan
   @clan ||= Clan.find(params[:clan_id])
 end

Great! But this is not easy to read, so what is this wizadry? 🧙‍♂️

Lets write the same method without the ||=

 
 def clan
    if @clan.present?
      @clan
    else
      @clan = Clan.find(params[:clan_id])
      @clan
    end
  end

Now this is more readable.

Essentially when the clan method gets called for the first time here users = clan.users the @clan varaible will be nil.

This means it will move to the else block and assign the @clan instance variable to the database record.

When it gets called a second time at render locals: {users: users, clan: clan} @clan will be present so it will just return @clan and not hit the database again.

Not only is this DRY, but it also means we don’t have to use a before_action and we learnt a cool way to utilise Memoization.

And they all lived happily ever after. The End. 👊

Co-authored with David Young