Question

My application has two associated models: Magazine and Article:

class Magazine < ActiveRecord::Base
  has_one :article
end

class Article < ActiveRecord::Base
  belongs_to :magazine
  validation_presence_of :title
end

From the Magazine show page I can create a new Article, so my routes.rb is configured like:

resources :magazines, :shallow => true do
  resources :articles
end

and in the Magazine show page I have the link “New article”, like:

<%= link_to 'New article', new_magazine_article_path(@article)

and an article helper to pass correct parameters to the form_for:

module ArticlesHelper
  def form_for_params
    if action_name == 'edit'
      [@article]
    elsif action_name == 'new'
      [@magazine, @article]
    end
  end
end

so I can use Article form_for like:

<%= simple_form_for(form_for_params) do |f| %> ...

The ArticlesController methods for new and create are:

respond_to :html, :xml, :js

def new
  @magazine = Magazine.find(params[:magazine_id])
  @article = Article.new
end

def create
  @magazine = Magazine.find(params[:magazine_id])
  @article = @magazine.build_article(params[:article])        
  if @article.save
    respond_with @magazine # redirect to Magazine show page
  else
    flash[:notice] = "Warning! Correct the title field."
    render :action => :new
  end
end

The problem occurs when there is a validation error with the title attribute, and the action new is rendered. In this moment I get the message: undefined method `model_name’ for NilClass:Class in the first line of form_for. I think it is because the @magazine parameter passed in the helper.

How could I solve this problem withou use redirect_to ? (I want to mantain the other attributes that were filled in the form .)

Answer 1

Your form_for_params method is returning nil, because the action_name is set to ‘create’, not ‘new’ or ‘edit’.

Try this:

elseif action_name == 'new' or action_name == 'create'
  [@magazine, @article]