Question

I have User, which can be one of three types: Admin, Student, Teacher. Everyone has other attributes. I am trying polymorphic association one-to-one like this:

User

class User < ActiveRecord::Base
    belongs_to :identity, :polymorphic => true
    accepts_nested_attributes_for :identity, :allow_destroy => true

    attr_accessible :email, :login, :remember_token, 
                    :password_confirmation, :password, :role
end

Student

class Student < ActiveRecord::Base
    attr_accessible :field

    has_one :user, :as => :identity
end

Controller

def new
     @user = User.new
end
def create
    @user = User.new(params[:user]) # It fails here.
    @user.identita.build
    ...
end

View

<%= form_for(@user) do |f| %>
    <%= f.label :login %><br />
    <%= f.text_field :login %>

    <%= f.fields_for [:identity, Student.new] do |i| %>  
    <%= i.label :field %><br />
    <%= i.textfield_select :field  %>
    <% end %>
<% end %>

When I submit this view (more complex, but this is the core), it sends hash like this:

{"utf8"=>"✓",
 "authenticity_token"=>"...",
 "user"=>{"login"=>"...",
 "student"=> {"field"=>"..."}
}

So it fails on marked line in controller with:

ActiveModel::MassAssignmentSecurity::Error in UsersController#create
Can't mass-assign protected attributes: student

What am I doing wrong? Something like :as=>”student” or twisting the realationship?

Answer 1

Firstly, fix:

<%= f.fields_for [:identity, Student.new] do |i| %>  

to:

<%= f.fields_for :identity, Student.new do |i| %>

Secondly, you are trying to use accepts_nested_attributes_for on a belongs_to relationship. This is not supported behavior AFAIK. Perhaps try moving that to the Student model:

class Student < ActiveRecord::Base
  attr_accessible :field

  has_one :user, :as => :identity
  accepts_nested_attributes_for :user, :allow_destroy => true
end

and make the view like this:

<%= form_for(Student.new) do |i| %>
  <%= i.fields_for :user, @user do |f| %>  
    <%= f.label :login %><br />
    <%= f.text_field :login %>
  <% end %>
  <%= i.label :field %><br />
  <%= i.textfield_select :field  %>
<% end %>