Association not created - ruby-on-rails

Question

This error has been troubling me for weeks. When I try to build an Item from a specified User and List, the Item is created, but the association, Wish is not.
If i try to do #item.lists.first.name it returns an error:
undefined method 'name' for nil:NilClass
I'm very new to rails, so I'm sure there is something that I have missed or misunderstood. Any help is thus much appreciated!
I have four models:
class User < ActiveRecord::Base
has_many :lists, dependent: :destroy
has_many :wishes, through: :lists
has_many :items, through: :wishes
class List < ActiveRecord::Base
belongs_to :user
has_many :wishes, dependent: :destroy
has_many :items, through: :wishes
class Wish < ActiveRecord::Base
belongs_to :list
belongs_to :item
class Item < ActiveRecord::Base
has_many :wishes
has_many :lists, through: :wishes
I want to create a new Item from lists_controller.rb show action:
class ListsController < ApplicationController
def show
#user = User.find(params[:user_id])
#list = #user.lists.find(params[:id])
#item = #list.items.build if current_user?(#user)
#items = #list.items
end
class ItemsController < ApplicationController
def create
#list = current_user.lists.find(params[:list_id])
#item = #list.items.build(params[:item])
if #item.save
flash[:success] = "Item created!"
redirect_to #item
else
render 'new'
end
end
My route file looks like this:
Wishlist::Application.routes.draw do
resources :users do
resources :lists, only: [:show, :create, :destroy]
end
resources :items, only: [:show, :new, :create, :destroy]
The form lists/show.html.erb:
<div class="modal-body">
<%= form_for(#item, html: { multipart: true }) do |f| %>
<div class="field">
<%= render 'items/fields', f: f %>
<%= hidden_field_tag :list_id, #list.id %>
</div>
</div>
and items/fields:
<%= render 'shared/error_messages', object: f.object %>
<%= f.label :image %>
<%= f.file_field :image %>
<%= f.label :remote_image_url, "or image URL" %>
<%= f.text_field :remote_image_url %>
<%= f.label :title %>
<%= f.text_field :title %>
<%= f.label :link %>
<%= f.text_field :link %>
Update
From the log:
Processing by ItemsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"pcDVdaDzZz4M17Kwjx8mKw6tTF9MjUvx1woTzaKRWJY=", "item"=>{"image"=>#<ActionDispatch::Http::UploadedFile:0x007fecdd1fd890 #original_filename="profile.jpg", #content_type="image/jpeg", #headers="Content-Disposition: form-data; name=\"item[image]\"; filename=\"profile.jpg\"\r\nContent-Type: image/jpeg\r\n", #tempfile=#<File:/var/folders/6y/j8zfcgmd02x5s439c0np8fjh0000gn/T/RackMultipart20130216-8413-3vzjuj>>, "remote_image_url"=>"", "title"=>"YES", "link"=>"SDFs"}, "list_id"=>"1", "commit"=>"Add"}
User Load (0.3ms) SELECT "users".* FROM "users" WHERE "users"."remember_token" = '5AXS8-7-YRyLyGDKYHIZRg' LIMIT 1
List Load (0.2ms) SELECT "lists".* FROM "lists" WHERE "lists"."user_id" = 1 AND "lists"."id" = ? LIMIT 1 [["id", "1"]]
(0.1ms) begin transaction
SQL (0.7ms) INSERT INTO "items" ("created_at", "image", "link", "title", "updated_at") VALUES (?, ?, ?, ?, ?) [["created_at", Sat, 16 Feb 2013 20:57:10 UTC +00:00], ["image", "profile.jpg"], ["link", "SDFs"], ["title", "YES"], ["updated_at", Sat, 16 Feb 2013 20:57:10 UTC +00:00]]
(2.7ms) commit transaction
Redirected to http://localhost:3000/items/1
Completed 302 Found in 830ms (ActiveRecord: 4.0ms)

Answer 1

This appears to be a known issue (but not really an issue) when using build on a has_many :through. You need to change the association for items on List to include inverse_of.
class List < ActiveRecord::Base
belongs_to :user
has_many :wishes, dependent: :destroy
has_many :items, through: :wishes, inverse_of: :wishes
end
It should then build the connecting Wish object for you properly.
See this relevant issue for more information: https://github.com/rails/rails/pull/7661#issuecomment-8614206

Answer 2

Let me try to summarize first: you have users, which have lists, which contain wishes, which are related to one particular item.
So let's have a look at the following line of code first:
#item.lists.first.name
So what happens is, that rails takes the item, goes through the wishes to all lists that contain this item, takes the first list and looks for the lists name. The problem you see occurs when the item is not related to any lists through wishes (e.g. because your create method for items does not connect new items to wishes and therefore the connection to the list is missing as well): So the item has no relation to a list and the result for #item.lists is an empty list. And well, the first element of an empty list is nothing (nil) and of course nil has no name.
The result is, what you see:
undefined method 'name' for nil:NilClass
One way to fix this, is to use try() which executes the related method only if it is available:
#item.lists.first.try(:name)
The other problem, you were talking about, is that no wish is generated, when you build an item from a user. I think you have to explicitly build the wish. I'm not 100% sure, but I think, declaring :through-relations in the model is only used for queries, not for generating/building.
Try something like the following in your items controller (The idea is to build the wish explicitly first, and after that build the item for the wish):
#wish = #list.wishes.build()
#wish.save
#item = #wish.item.build(params[:item])

Answer 3

After lots of googling, and trial and error, I found that I had to build the Wish explicitly. The create action ended up looking like this:
def create
#list = current_user.lists.find(params[:list_id])
#item = Item.create!(params[:item])
#item.wishes.build(list_id: #list.id)
if #item.save
flash[:success] = "Item created!"
redirect_to #item
else
render 'new'
end
end