Rails 4 form_for double nested comments - ruby-on-rails

Question

First of all, I've found some very helpful answers already (see links below), but I'm still struggling to make this work. I haven't been programming for long, and this is definitely stretching what I feel comfortable claiming to understand.
I have three models. For example's sake, I'll call them Tree, Bark, Engraving (the comments). I'm trying to add a simple form partial for users to submit new comments (engravings).
The below setup is producing the error:
NoMethodError at /trees/1
undefined method `tree' for nil:NilClass
Controllers
class TreesController < ApplicationController
def show
#barks = Bark.where(tree_id: #tree.id)
#engraving= Engraving.new( :bark=> #bark)
end
end
class BarksController < ApplicationController
def show
#engravin= Engraving.new( :bark=> #bark)
#engravings = Engraving.where(bark_id: #bark.id)
end
end
class EngravingsController < ApplicationController
def show
#bark= Bark.find(params[:bark_id])
#engraving = Engraving.new(params[:engraving])
end
end
def create
#bark = Bark.find(params[:bark_id])
#engraving = Engraving.new(params[:engraving])
#engraving.user_id = current_user.id
respond_to do |format|
if #engraving.save
format.html { redirect_to tree_bark_path(#tree, #bark), notice: 'Comment was successfully created.' }
format.json { render action: 'show', status: :created, location: #engraving}
else
format.html { render action: 'new' }
format.json { render json: #engraving.errors, status: :unprocessable_entity }
end
end
end
Models
class Tree < ActiveRecord::Base
belongs_to :user
has_many :barks
has_many :engravings, through: :barks
end
class Bark< ActiveRecord::Base
belongs_to :user
belongs_to :tree
has_many :engravings
delegate :title, :link, :to => :tree, :prefix => true
def new_engraving_path
Rails.application.routes.url_helpers.new_tree_bark_engraving_path(book, self)
end
def serializable_hash(*args)
super.merge 'new_engraving_path' => new_engraving_path
end
end
class Engraving< ActiveRecord::Base
belongs_to :user
belongs_to :bark
delegate :tree, :to => :bark, :prefix => true
def to_edit_path
Rails.application.routes.url_helpers.edit_tree_bark_engraving_path bark_tree, bark, self
end
def to_path
Rails.application.routes.url_helpers.tree_bark_engraving_path bark_tree, bark, self
end
def serializable_hash(*args)
super.merge 'path' => to_path, 'edit_path' => to_edit_path
end
end
Calling the Form in a View
On the Tree show page, I have all the Bark populating there. Clicking on an Bark element opens a modal, where the Bark.content and Engravings are shown. Under the last Engraving, I want to have a text_area form to create a new Engraving, and add it to this "piece" of bark.
# trees\show.html.erb
<%= render #barks %>
# barks\_bark.html.erb
<%= render partial: 'barks/modal', locals: { bark: bark} %>
# barks\modal.html.erb
<% render partial: '/engravings/simpleForm', :collection => bark.engravings %>
# Form partial
<%= form_for #engraving, :url => (new_tree_bark_engraving_path(:tree_id => #bark.tree, :bark_id => #bark.id)) do |f| %>
<%= f.hidden_field :bark_id %>
<div class="field">
<%= f.text_area :content %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
Routes
get "pages/home"
resources :trees do
collection do
get 'search'
post 'load_from_api'
end
resources :barks do
resources :engravings
end
end
resources :barks do
resources :engravings
end
root "trees#index"
Other Answer Sources
Rails Routing Error for nested form_for
Twice Nested Resource Form_For Issue
form_for with nested resources

Answer 1

Your error message NoMethodError at /trees/1 undefined method 'tree' for nil:NilClass means that somewhere you must be calling .tree on a nil object. The only place I can see that happening is in the form partial. I'm assuming that #bark.tree is failing.
From your question it is not clear which controller you are rendering the partial from, but the most likely cause is that #bark is not being passed into the form. Make sure that you are passing #bark into your partial, here is an example:
<%= render partial: "form", locals: {bark: #bark} %>

Answer 2

You're definining #barks in your trees#show action, but never #bark
def show
#barks = Bark.where(tree_id: #tree.id)
#engraving= Engraving.new( :bark=> #bark)
end
So #bark is nil.