Error with initial calculation of instance variable values in Rails controller - ruby-on-rails

Question

I'm looking to build a basic app that provides a score based on how enjoyable users found a given lunch.
In my view, the voting buttons look like this:
<%= form_for :lunch, url: upvote_lunch_lunch_path(params[:id]), method: :put, :html => {:class => 'form-inline'} do |f| %>
<%= f.hidden_field :liked %>
<%= f.hidden_field :id %>
<%= f.submit "Like Lunch", class: "btn btn-large btn-success" %>
<% end %>
<%= form_for :lunch, url: downvote_lunch_lunch_path(params[:id]), method: :put, :html => {:class => 'form-inline'} do |f| %>
<%= f.hidden_field :disliked %>
<%= f.hidden_field :id %>
<%= f.submit "Dislike Lunch", class: "btn btn-large btn-danger" %>
<% end %>
Displaying the value (in a partial) looks like this:
<%= number_to_percentage(#enjoy_score * 100, precision: 0, format: "%n") %>
and finally, in my "lunches" controller, I have the following:
def show
#lunch = Lunch.find(params[:id])
#provider = #lunch.provider
#total_enjoy_votes = (#lunch.liked + #lunch.disliked)
#enjoy_score = (#lunch.liked.to_f / #total_enjoy_votes)
end
def downvote_lunch
#lunch = Lunch.find(params[:id])
#lunch.increment!(:disliked)
redirect_to #lunch
end
def upvote_lunch
#lunch = Lunch.find(params[:id])
#lunch.increment!(:liked)
redirect_to #lunch
end
Everything works as expected, so long as the database already has values for liked and disliked against that particular lunch ID. If, for example, you are the first person to attempt to answer this question (lets say just after a lunch was created) the application errors out with the message "undefined method +' for nil:NilClass" at this line:#total_enjoy_votes = (#lunch.liked + #lunch.disliked)`
Two questions:
Why is it if I open the database (with sqlitebrowser) and "seed" a row of data with liked = 1 and disliked = 1 the methods will work as expected, but if I don't, it errors out? What should I do to "initialize" #lunch.liked and #lunch.disliked so that there isn't an error for the initial user?
(Bonus Point) How can I keep the controller code DRY so I don't have to type #lunch = Lunch.find(params[:id])
at the beginning of every method?
Thanks so much in advance. I apologize if this is a insanely simple question.

Answer 1

1.
#total_enjoy_votes = (#lunch.liked + #lunch.disliked) errors, because #lunch.liked is nil, since it is never set to anything, as well, as #lunch.disliked
To avoid this error you should check if liked and disliked are present.
liked = #lunch.liked ? #lunch.liked : 0
disliked = #lunch.disliked ? #lunch.disliked : 0
#total_enjoy_votes = (liked + disliked)
#enjoy_score = (liked.to_f / #total_enjoy_votes)
2.
before_filter :find_lunch!, only: [ :update, :destroy ] # list of actions where to perform the method.
private
def find_lunch!
#lunch = Lunch.find(params[:id])
end
EDIT
explanation to the line: #lunch.liked ? #lunch.liked : 0
It is ternary operator which comes in handy really often.
Syntax
boolean_expression ? true_expression : false_expression
Example
grade = 88
status = grade >= 70 ? "pass" : "fail"
#=> pass
It is the same, as if I wrote something like:
if #lunch.liked.nil?
liked = 0
else
liked = #lunch.liked
end

Answer 2

First approach:-
def show
#lunch = Lunch.find(params[:id])
#provider = #lunch.provider
liked = ((#lunch.liked.present? and #lunch.liked >= 0) ? #lunch.liked : 0)
#total_enjoy_votes = liked + ((#lunch.disliked.present? and #lunch.disliked >= 0) ? #lunch.disliked : 0)
#enjoy_score = (#total_enjoy_votes > 0 and liked > 0) ? (#lunch.liked.to_f / #total_enjoy_votes) : 0
end
Second approach:-
In Lunch model set a calback as
class Lunch < < ActiveRecord::Base
before_save :initialize_liked_disliked
private
def initialize_liked_disliked
if self.new_record?
self.liked = 0
self.disliked = 0
end
end
end
It will initialize both the fields with 0, then make the changes in controller as:
def show
#lunch = Lunch.find(params[:id])
#provider = #lunch.provider
#total_enjoy_votes = #lunch.liked + #lunch.disliked
#enjoy_score = (#total_enjoy_votes > 0) ? (#lunch.liked.to_f / #total_enjoy_votes) : 0
end