Question

What is the right way to create a data model object so that I can create a form. When I first create the object it doesn’t have params data with the valid attribute data so the build_ returns an invalid object and rails throws 500 Internal Server Error.

Here’s what I’m looking at:

Models

class Credentials < ActiveRecord::Base
belongs_to user
...
class User < Active ::Base
has_one :credentials, :dependent => :destroy
...

Controller

def new
@cred = current_user.build_credentials 
    render 'edit'
end
    ...

View

<%= form_for(@cred) do |f| %>

Log

Started GET "/credentials" for 127.0.0.1 at 2012-11-16 00:43:26 -0500
Processing by CredentialsController#new as HTML
  [1m[35mUser Load (0.3ms)[0m  SELECT `users`.* FROM `users` WHERE `users`.`remember_token` = 'oWGXC2dmcg2sRyg5mvA6fw' LIMIT 1
  [1m[36mUser Load (0.3ms)[0m  [1mSELECT `users`.* FROM `users` WHERE `users`.`id` = 1 LIMIT 1[0m
  [1m[35mCredentials Load (0.4ms)[0m  SELECT `credentials`.* FROM `credentials` WHERE `credentials`.`user_id` = 1 LIMIT 1
  [1m[36m (0.1ms)[0m  [1mBEGIN[0m
  [1m[35m (0.1ms)[0m  COMMIT
  Rendered credentials/edit.html.erb within layouts/application (8.8ms)
Completed 500 Internal Server Error in 100ms

rsec results

undefined method `credentials_index_path' for #<#<Class:...

The credentials model has many verification requirements so the object that is being created by new is invalid (I can confirm this in the console) as the only value that is populated is the foreign key, in this case user_id. However in the console I can establish the @credentials variable and the in the next command assign all the attributes. In the web server the 500 error makes it impossible to follow the same approach.

Update
To make this more clear, the problem is that the @cred = current_user.build_credentials is returning a nil.
I put an raise @cred.inspect call directly after creating @cred to validate that its nil. When I get to the view form_for(@cred)... @cred is just a nil object so it pukes with undefined method 'model_name' for NilClass:Class

Answer 1

Why Credentials belongs to controller? It should belongs to user, and if you use has_one association, you should use singular noun of credential:

Model

class Credential < ActiveRecord::Base
belongs_to user
...
class User < Active ::Base
has_one :credential, :dependent => :destroy

Controller

In your controller, if you want create new object for form, you only need:

def new
  @cred = current_user.build_credential 
end

def create
  @cred = current_user.build_credential(params[:cred])
  if @cred.save
    # what you do when creating object success
  else
    # what you do when creating object failed
  end
end

View

In your new.html.erb in app/views/credentials/ folder, create your form:

<%= form_for @cred do |f| %>
...
<% end %>

To link to new.html.erb page, in your view create a link_to:

<%= link_to 'New Credential', new_credential_path %>

Routes

If you want to use RESTful route, In your routes.rb, add resources for credential you don’t have:

resources :credentials