Ruby on Rails : undefined method `-' for nil:NilClass - ruby-on-rails

Question

I'm trying to add save_with_initial_vote method to my Post class. It should both save the post and create its vote inside a transaction.
My post.rb looks like this:
class Post < ActiveRecord::Base
has_one :summary
has_many :comments, dependent: :destroy
has_many :votes, dependent: :destroy
belongs_to :user
belongs_to :topic
mount_uploader :image, ImageUploader
def up_votes
votes.where(value: 1).count
end
def down_votes
votes.where(value: -1).count
end
def points
votes.sum(:value)
end
def update_rank
age_in_days = (created_at - Time.new(1970,1,1)) / (60 * 60 * 24) # 1 day in seconds
new_rank = points + age_in_days
update_attribute(:rank, new_rank)
end
default_scope { order('rank DESC') }
scope :ordered_by_title, -> { order('posts.title ASC') }
scope :ordered_by_reverse_created_at, -> { order('posts.created_at DESC') }
validates :title, length: { minimum: 5 }, presence: true
validates :body, length: { minimum: 20 }, presence: true
validates :topic, presence: true
validates :user, presence: true
def markdown_title
render_as_markdown(self.title)
end
def markdown_body
render_as_markdown(self.body)
end
def create_vote
user.votes.create(value: 1, post: self)
end
def save_with_initial_vote
ActiveRecord::Base.transaction do
save
user.votes.create(value: 1, post: self)
end
end
private
def render_as_markdown(markdown)
renderer = Redcarpet::Render::HTML.new
extensions = {fenced_code_blocks: true}
redcarpet = Redcarpet::Markdown.new(renderer, extensions)
(redcarpet.render markdown).html_safe
end
end
My posts_controller.rb looks like this:
class PostsController < ApplicationController
def show
#topic = Topic.find(params[:topic_id])
authorize #topic
#post = Post.find(params[:id])
#comments = #post.comments
end
def new
#topic = Topic.find(params[:topic_id])
#post = Post.new
authorize #post
end
def edit
#topic = Topic.find(params[:topic_id])
#post = Post.find(params[:id])
authorize #post
end
def create
#topic = Topic.find(params[:topic_id])
#post = current_user.posts.build(post_params)
authorize #post
if #post.save_with_initial_vote
flash[:notice] = "Post was saved."
redirect_to [#topic, #post]
else
flash[:error] = "There was an error saving the post. Please try again."
render :new
end
end
def update
#topic = Topic.find(params[:topic_id])
#post = Post.find(params[:id])
authorize #post
if #post.update_attributes(post_params)
flash[:notice] = "Post was updated."
redirect_to [#topic, #post]
else
flash[:error] = "There was an error saving the post. Please try again."
render :new
end
end
def destroy
#topic = Topic.find(params[:topic_id])
#post = Post.find(params[:id])
authorize #post
if #post.destroy
flash[:notice] = "\"#{#post.title}\" was deleted."
redirect_to #topic
else
flash[:error] = "There was an error deleting your post. Please try again."
render :show
end
end
private
def post_params
params.require(:post).permit(:title, :body, :image)
end
end
My vote.rb looks like this:
class Vote < ActiveRecord::Base
belongs_to :user
belongs_to :post
validates :value, inclusion: {in: [-1, 1], message: "%{value} is not a valid vote." }
after_save :update_post
private
def update_post
post.update_rank
end
end
When I do, I get an error:
undefined method `-' for nil:NilClass

Answer 1

To troubleshoot your problem start by looking at the source for your error. The error says "undefined method '-' for nil:NilClass".
What this means is that you are trying to subtract from an object that is in fact Nil.
When you try to save a Vote, in vote.rb model you call update_post after save. This calls the update_rank method in the Post model. This method then subtracts a Time object from 'created_at'. The problem thus lies with created_at returning a nil value.
You can try using pry gem to debug your code better. You can then add 'binding.pry' anywhere in your model/controller code and step through each line to see what is the value of each variable at that instant.