Creating separate models in single form with accepts_nested_attributes_for (b/t & has_many)

Question

Trying to use nested attributes to create an Organization and a User in one form. I'm getting the following error: Can't mass-assign protected attributes: user (ActiveModel::MassAssignmentSecurity::Error)

Pulling my hair out on this one

organization.rb

class Organization < ActiveRecord::Base  
  attr_accessible :name, :users_attributes
  has_many :users, dependent: :destroy      
  accepts_nested_attributes_for :users  
end

user.rb (using devise)

class User < ActiveRecord::Base
  attr_accessible :email     
  belongs_to :organization
end

new.html.haml

= form_for @organization do |f|

  = f.label :name, "Company Name"
  = f.text_field :name, placeholder: "Company Name"

  = f.fields_for :user do |ff| -# tried :users here and the form doesn't render
    = ff.label :email, "Email Address"
    = ff.email_field :email, placeholder: "Email Address"

= f.submit "Create Account"

Answer 1

This has been answered a lot on Stackoverflow.

f.fields_for :users

And in the controller, you need to build a user:

@organization.users.build

You get Can't mass-assign protected attributes: user because the user attribute is not accessible, because it doesn't exist.