Calling Document.find with nil is invalid MongoId in Rails 4 - mongodb

Question

There are three models in my app: User, Subject and Note.
I've already embedded Note model into Subject model and it works. Now when I try to embed Subject into User always get Mongoid::Errors::InvalidFind coming from this line of code:
In the method one should define to use in before_action at the beginning of the controller
#user = User.find(params[:user_id])
subjects_controller
class SubjectsController < ApplicationController
#before_filter :authenticate_user!, only: [:index]
before_action :set_subject, only: [:show, :edit, :update, :destroy]
before_action :load_user
def index
#subjects = #user.subjects
end
def new
#subject = #user.subjects.build
end
def show
end
def create
#subject = #user.subjects.create(subject_params)
respond_to do |format|
if #subject.save
format.html {redirect_to subject_path(#subject)}
else
format.html {render 'new'}
end
end
end
def update
if #subject.update(subject_params)
redirect_to #subject
else
render 'edit'
end
end
def delete
end
private
def subject_params
params.require(:subject).permit(:name)
end
def set_subject
#subject = #user.subject.find(params[:id])
end
def load_user
#user = User.find(params[:user_id])
end
end
Routes
resources :users do
resources :subjects do
resources :notes
end
end
Right now I'm pretty stuck here because haven't found a way to work this around, hope someone around can give a hand.

Answer 1

Take into account this RoR best practice "Resources should never be nested more than 1 level deep." http://guides.rubyonrails.org/routing.html#nested-resources
A collection may need to be scoped by its parent, but a specific member can always be accessed directly by an id, and shouldn’t need scoping (unless the id is not unique, for some reason).
http://weblog.jamisbuck.org/2007/2/5/nesting-resources