Ruby on Rails: undefined method `' for #<#<Class:>> - ruby-on-rails

Question

I'm relatively new to rails and I've been struggling with this for a couple of days. I'd be much appreciated if you can see where I've gone wrong.
When I view the page in the web browser I get the following message:
Showing C:/Users/Matt/Documents/GitHub/Outputer/app/views/studies/index.html.erb where line #8 raised:
undefined method `studies_path' for #<#:0x6b03808>
8: <%= form_for #new_study do |f| %>
studies_controller:
def index
#line = current_user.lines.find_by_id(params[:line_id])
#machine = #line.machines.find_by_id(params[:machine_id])
#studies = #machine.studies.paginate(page: params[:page], :per_page => 10)
#new_study = #machine.studies.build
end
def create
#study = current_user.lines.machines.study.build(params[:study])
if #study.save
flash[:success] = "Study created"
else
flash[:error] = "Error : Invalid study description"
end
redirect_to :back
end
index.html
....
<section>
<%= form_for #new_study do |f| %>
<div class="field">
<%= f.text_field :description, placeholder: "New study description..." %>
</div>
<%= f.submit "Create", class: "btn" %>
<% end %>
</section>
....
Study Model
....
class Study < ActiveRecord::Base
belongs_to :machine
belongs_to :line
attr_accessible :avg_speed, :avg_uptime, :avg_yield, :description, :duration, :is_active, :start_time, :stop_time, :line_id
validates ....
has_many :events, dependent: :destroy
....
end
....
rake routes:
....
save_line_machine_study PUT /lines/:line_id/machines/:machine_id/studies/:id/save(.:format) studies#save {:has_many=>:machines}
line_machine_studies GET /lines/:line_id/machines/:machine_id/studies(.:format) studies#index {:has_many=>:machines}
POST /lines/:line_id/machines/:machine_id/studies(.:format) studies#create {:has_many=>:machines}
new_line_machine_study GET /lines/:line_id/machines/:machine_id/studies/new(.:format) studies#new {:has_many=>:machines}
edit_line_machine_study GET /lines/:line_id/machines/:machine_id/studies/:id/edit(.:format) studies#edit {:has_many=>:machines}
line_machine_study GET /lines/:line_id/machines/:machine_id/studies/:id(.:format) studies#show {:has_many=>:machines}
PUT /lines/:line_id/machines/:machine_id/studies/:id(.:format) studies#update {:has_many=>:machines}
DELETE /lines/:line_id/machines/:machine_id/studies/:id(.:format) studies#destroy {:has_many=>:machines}
....
routes.rb
resources :users
resources :lines, :has_many => :machines, only: [:index, :edit, :destroy, :show, :create] do
resources :machines, only: [:new, :create, :edit, :update] do
resources :studies
end
end
If I remove the form the page works fine which would suggest its in the form. I've tested the controller commands in the console and they all appear fine - I can create a new study object.
Thanks in anticipation

Answer 1

When you use form_for with a model instance, it defaults to the POST action for that controller which would be your studies_path. This is usually mapped to create in the controller.
From the looks of it, you need to add a route in routes.rb to handle that post request (see resources). You will also need a create method in your studies controller.
Here is a good guide for learning the basics of routing in rails.

Answer 2

Although a missing route is the most common reason for that (not-very-helpful) error, it can also be raised if one or both sides of a has_many/belongs_to relationship is missing or is incorrectly defined. Another place to look is a form field for an attribute that doesn't exist in the related model.

Answer 3

<%= form_for #new_study %> is equivalent to <%= form_for #new_study, url: studies_url %>. As your routes are defined differently, you need to pass the url you'd like to submit the form to to the url parameter (find form_for in the Rails API docs to see what other options it takes).
Three level deep nesting is kind of ugly to maintain, so I'd suggest the following:
resources :users
resources :lines do
resources :machines
end
resources :machines do
resources :studies
end
These shallow routes are much nicer to maintain. There's also a shallow: true option on nested resources calls, see the docs.
In your case:
# With the current setup
<%= form_for #new_study, url: line_machine_studies_path(#line, #machine)
# Same, my preference
<%= form_for [#line, #machine, #new_study] %>
# If you make your routes shallow,
# #line is not nescessary, as #machine holds all information about associations
<%= form_for #new_study, url: machine_studies_path(#machine) %>
# Same, my preference, what I would do
<%= form_for [#machine, #new_study] %>
General suggestions:
#study is preferred over #new_study. #study.new_record? will tell you whether the object is a new record if you need.
There's no has_many :... option on resources routes as far as I'm aware
Google rails shallow routes for more info. Keep nesting to two levels. Think about only what information you really require when creating objects and keep the URLs and url helpers as slim as possible.