ruby on rails - App works fine on Localhost, but has NoMethodError (undefined method `-' for nil:NilClass) on Heroku -

i new ruby , experiencing error after deploying app heroku.

i test app on localhost , works fine; however, when try save new post, have "undefined method `-' nil:nilclass" on heroku. have searched solutions on stackoverflow far haven't found anything.

error got heroku logs:

2015-04-01t03:37:23.412066+00:00 app[web.1]: nomethoderror (undefined method `-' nil:nilclass): 2015-04-01t03:37:23.412076+00:00 app[web.1]:   app/controllers/topics/posts_controller.rb:22:in `create' 2015-04-01t03:37:23.412074+00:00 app[web.1]:   app/models/post.rb:37:in `save_with_initial_vote' 2015-04-01t03:37:23.407819+00:00 app[web.1]:    (1.0ms)  rollback 2015-04-01t03:37:23.412070+00:00 app[web.1]:   app/models/vote.rb:20:in `update_post' 2015-04-01t03:37:23.412078+00:00 app[web.1]:  2015-04-01t03:37:23.412063+00:00 app[web.1]:  2015-04-01t03:37:23.412068+00:00 app[web.1]:   app/models/post.rb:30:in `update_rank' 2015-04-01t03:37:23.412072+00:00 app[web.1]:   app/models/post.rb:39:in `block in save_with_initial_vote' 

from logs, looks problem update_rank method in post.rb.

from app/models/post.rb:

  class post < activerecord::base   has_many :comments, dependent: :destroy   has_many :votes, dependent: :destroy   has_many :favorites, dependent: :destroy   belongs_to :user   belongs_to :topic    default_scope { order('rank desc') }   scope :visible_to, ->(user) { user ? : joins(:topic).where('topics.public' => true) }    validates :title, length: { minimum: 5 }, presence: true   validates :body, length: { minimum: 20 }, presence: true   validates :topic, presence: true   validates :user, presence: true      def update_rank         age_in_days = (created_at - time.new(1970,1,1)) / (60 * 60 * 24)         new_rank = points + age_in_days          update_attribute(:rank, new_rank)       end        def save_with_initial_vote         activerecord::base.transaction           post = post.create           user.votes.create(value: 1, post: self)         end       end 

from app/controllers/topics/posts_controller.rb:

  def create     @topic = topic.find(params[:topic_id])     @post = current_user.posts.build(post_params)     @post.topic = @topic         authorize @post      if @post.save_with_initial_vote       flash[:notice] = "post saved."       redirect_to [@topic, @post]     else       flash[:error] = "there error saving post. please try again."       render :new     end   end 

models/vote.rb:

after_save :update_post    def up_vote?     value == 1   end    def down_vote?     value == -1   end    def update_post     post.update_rank   end 

i have run "heroku rn db:migrate" , "heroku restart" error still exists. rails version 4.2.0 , ruby version 2.0.0.

does know cause , how make work on heroku?

the use of "-" in code provided in first line of post#update_rank

note, when run following in irb

irb> (nil - time.new(1970,1,1)) / (60  * 60 * 24)  

i following error.

nomethoderror: undefined method `-' nil:nilclass 

remember in ruby class. many classes implement operator methods. datetime class does, nilclass not.

i suspect created_at not yet set , @ point nil value.