I cannot seem to get a nested form to generate in a rails view for a belongs_to relationship using the new accepts_nested_attributes_for facility of Rails 2.3. I did check out many of the resources available and it looks like my code should be working, but fields_for explodes on me, and I suspect that it has something to do with how I have the nested models configured.

The error I hit is a common one that can have many causes:

'@account[owner]' is not allowed as an instance variable name

Here are the two models involved:

class Account < ActiveRecord::Base
  # Relationships
  belongs_to :owner, :class_name => 'User', :foreign_key => 'owner_id'
  accepts_nested_attributes_for :owner
  has_many :users
end

class User < ActiveRecord::Base
  belongs_to :account
end

Perhaps this is where I am doing it 'rong', as an Account can have an 'owner', and may 'users', but a user only has one 'account', based on the user model account_id key.

This is the view code in new.html.haml that blows up on me:

- form_for :account, :url => account_path do |account|
  = account.text_field :name
  - account.fields_for :owner do |owner|
    = owner.text_field :name

And this is the controller code for the new action:

class AccountsController < ApplicationController
  # GET /account/new
  def new
    @account  = Account.new
  end
end

When I try to load /account/new I get the following exception:

NameError in Accounts#new
Showing app/views/accounts/new.html.haml where line #63 raised:
@account[owner] is not allowed as an instance variable name

If I try to use the mysterious 'build' method, it just bombs out in the controller, perhaps because build is just for multi-record relationships:

class AccountsController < ApplicationController
  # GET /account/new
  def new
    @account  = Account.new
    @account.owner.build
  end
end

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.build

If I try to set this up using @account.owner_attributes = {} in the controller, or @account.owner = User.new, I'm back to the original error, "@account[owner] is not allowed as an instance variable name".

Does anybody else have the new accepts_nested_attributes_for method working with a belongs_to relationship? Is there something special or different you have to do? All the official examples and sample code (like the great stuff over at Ryans Scraps) is concerned with multi-record associations.