Question

I have a problem with subclassing Rail’s models. Suppose you have a User model and several subclasses of it (user types) that store specific methods and associations, for example: Director, Admin, Trainee, Instructor, etc. This is just simple “single-table inheritance”. The problem is 2-fold.

1. paths/urls often crash or do weird things when you pass a subclass rather than the base class. Here’s an example:

   <% if user.enabled? %>
     <%= link_to 'Disable', disable_user_path(user) %>
   <% else %>
     <%= link_to 'Enable', enable_user_path(user) %>
   <% end %>
   

   If you pass in a User model, it works just fine. But if you pass a subclass, like Admin, it throws this exception:

   No route matches {:controller=>"users", :action=>"disable", :id=>#<Admin id: 1, first_name: "Ken", ..., created_at: "2011-05-23 21:01:35", updated_at: "2011-05-23 21:04:28">}
   

   Clearly, this is not behaving correctly. How can we get rails to use the base class all the time?

2. Even more disturbing is forms (I am using simple_form). Let’s say you have a /profile form. You want all User subclasses to access it equally, and you don’t want to deal with their subclasses on a special-case basis; it should be 100% generic.

   For some reason, if the user is an Admin, it will post the params hash as

   params[:admin]
   

   Even worse, if you view the source of the form, it actually says user[first_name] instead of admin[first_name], so something is definitely screwy! The instance variable is @user too, so I don’t see why it should be posting to params[:admin].

Here is the form view code for (2):

<%= simple_form_for(@user, :url => profiles_path, :method => :put, :html => {:multipart => true}) do |f| %>
  <%= f.error_notification %>

  <div class="form">
    <fieldset>
      <legend>Personal Information</legend>
      <%= f.input :first_name %>
      <%= f.input :last_name %>
    </fieldset>

    <fieldset>
      <legend>Credentials</legend>
      <%= f.input :email %>
    </fieldset>

    <fieldset>
      <legend>Preferences</legend>
      <%= f.input :receive_email_notifications %>
      <%= f.input :receive_newsletters %>
      <%= f.input :allow_private_messages %>
    </fieldset>

    <fieldset>
      <legend>Avatar</legend>
      <p>
          Select an image from your computer to use as your avatar. You will be given the oppurtunity
          to crop this image further after your image has been uploaded.
      </p>
      <%= f.input :avatar, :as => :file, :label => "Select File" %>
      <%= f.hidden_field :avatar_cache %>
    </fieldset>

    <div class="actions">
      <%= f.button :submit, :value => "Update Profile" %>
      <%= link_to 'Cancel', profiles_path %>
    </div>
  </div>

<% end %>

Here are the controller actions to first render the view and when I submit the form:

  def edit
    @user = User.find(current_user.id)
  end

  def update
    @user = User.find(current_user.id)

    if @user.update_attributes(params[:user]) # <-- this bombs for Admin subclass
      if params[:user][:avatar] # <-- this would bomb also.
        redirect_to(crop_avatar_profiles_path)
      else
        redirect_to(profiles_path, :notice => 'Your profile was successfully updated.')
      end
    else
      render :action => "edit"
    end
  end

  def crop_avatar
    @user = User.find(current_user.id)
  end

  def update_avatar
    @user = User.find(current_user.id)
    @user.crop(params[:x].to_i, params[:y].to_i, params[:h].to_i, params[:w].to_i)

    redirect_to(profiles_path, :notice => 'Your profile and avatar was successfully updated.')
  end

Besides thinking up some pretty inelegant solutions (especially to the forms problem), I am at a loss as to how I can fix them. There has to be a better way to deal with these situations.

Answer 1

I had the same problem with STI due to the dynamic casting rails did for me. I solved it using the becomes method.

In your User model:

    #return an array [User, User, ...], instead of [Admin, Director, Director, Instructor, ...]
    def self.all_without_typecast
      self.all.collect! do |u|
          u.becomes(User)
      end
    end

    #makes subclasses of User (Admin, Director, ...) into User class
    def userize #cast_into_user could be a better name
      self.becomes(User)
    end

Now whenever you don’t want a user instance to be of an specific type (Admin, Director..) but just User, you can do:

    @user = @user.userize

And your @user object will now be treated as a User, not Admin or Director or whatever, same goes for all_without_typecast, like this:

    @users = User.all_without_typecast # if you're having problems with User.all

Maybe this methods could be run before each scope so you don’t have to rewrite every one of them.
Hope it helps!