Why do I get "undefined method 'model_name' "?

Question

I'm doing a small experiment and trying to extract some logic from controller's action.

I have code like this:

Controller

def create

  @user = User.new(user_params)

  if @user.save 

    redirect_to user_url(@user), notice: 'Welcome to MyApp' 

  else

    render :new

  end

end

And try to create something like this:

Controller

def create

  user_service.display    

end



private 



def user_service

  RenderService.new(self)

end

Separate Service object

require 'forwardable'



class RenderService

  extend Forwardable

  delegate [:params, :user_url, :redirect_to, :render] => :@obj



  attr_reader :obj, :user

  def initialize(obj)

    @obj  = obj

    @user = User.new(user_params)

  end



  def display 

    redirect_to(user_url(user), notice: 'Welcome to MyApp')    if user.save

    render(:new)                                               unless user.save

  end



  private 



  def user_params

    params.require(:user).permit(:name, :email, :password, :confirmation)

  end

end

Form for creating new user

= simple_form_for(@user, html: { }) do |form| 

  = form.input :name, required: true

  = form.input :email, required: true

  = form.input :password, required: true 

  = form.input :confirmation, required: true

  = form.submit 'Create my account', class: 'btn btn-primary'

As you can see all I tried to do is to encapsulate logic into a separate class, but I got an error from

ActionView::Template::Error

for some reason and the error says

undefined method 'model_name' for nil:NilClass

.

I really don't understand why it does not work. To me it looks like I sent the same message to the same object and it should work just fine, but it doesn't.

Second question: why is

ActionView

involved here?

Thank you for any explanation.

Oh, by the way, if you know the code responsible for such behavior and where it lives in the Rails repository please point me to it.

Thanks in advance. :)

Answer 1

This happens because you're setting the @user instance variable on the instance of RenderService rather than on the controller instance (@obj).

render creates an instance of ActionView::Base (self in a template) and then copies the controller's instance variables to the view instance. Since @user isn't set on the controller instance, it's never copied and no such variable is set in the view. The view calls model_name on the nonexistent @user instance variable and you get the error you saw.

You should be able to fix it by setting @user on the controller instance. Instead of

@user = User.new(user_params)

do

@obj.instance_variable_set :@user, User.new(user_params)

It's difficult to fully describe what happens when a Rails controller renders a view, because there is a lot of subclassing and overridden methods, so if you really want to understand it I recommend stepping through a render call in a debugger. Eventually you'll get to ActionView::Rendering#_render_template and then #view_context. There you'll see the call to #view_assigns, where the controller tells the view what instance variables it has.

Why do I get "undefined method 'model_name' "?

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