Question

I’ve run into an error in my application, to explain the context: a user has several cars, each car can make a tanking therefore my TankingLogsController is the following

class TankingLogsController < ApplicationController
    def new
        @user = User.find(params[:user_id])  
        @car = @user.cars.find(params[:car_id])
        @tankinglog = @cars.tanking_logs.build
    end
end

This is my routes.rb file

Estaciones::Application.routes.draw do
root :to => "static_pages#home"
match '/contact', :to=>'static_pages#contact'
match '/about', :to=>'static_pages#about'
devise_for :users
resources :users do
    resources :cars do
    resources :tanking_logs
end
end
...

I encountered this error:

NoMethodError in TankingLogsController#new

undefined method `tanking_logs' for nil:NilClass

Application Trace | Framework Trace | Full Trace
app/controllers/tanking_logs_controller.rb:5:in `new'

Why is my application failing?

Answer 1

You have a very obvious error, and Ruby is telling you exactly what it is and where to find it.

You’ve written:

@car = @user.cars.find(params[:car_id])
@tankinglog = @cars.tanking_logs.build

You initialize a variable called @car on the first line, and you reference a second uninitialized variable @cars on the second line.

You have never initialized the @cars variable, so it is nil. Calling @cars.tanking_logs causes undefined method 'tanking_logs' for nil:NilClass, because you’re effectively writing nil.tanking_logs.

The second line should be using @car.tanking_logs.