Problems without using instance variables to hand data from a Rails controller to a view

1-problem-without-instance-variables.md

This is the first time I've run into an issue like this.

I've been using focused_controller in some projects and in others I've just extracted the expose method (see below). Everything was always fine.

Now in a CRM app I have a CommentsController that handles comments for multiple commentable resources – e.g. a contact. The comment form is currently always displayed on the resource's show page. So in case of errors (e.g. empty comment text), the commentable's show template is the one that needs to be re-rendered in order to properly show error messages. I don't want to do ugly stuff like redirect with error messages in the GET parameters or put something in the session.

As it stands, I found two solutions that actually work and allow me to use contact (as a method call) in the show template:

• Use a combination of instance_variable_set/get/defined?. Remember: the commentable is essentially polymorphic so I have to set the name dynamically.
• Pass a :locals hash to the render call.

I don't really mind the :locals approach – but at the same time I feel this should be easier – e.g. by using an instance-level expose call.

2-expose.rb

# thanks, @jonleighton
# https://github.com/jonleighton/focused_controller/blob/master/lib/focused_controller/mixin.rb#L24
module Expose
  extend ActiveSupport::Concern

  module ClassMethods
    def expose(name, &block)
      if block_given?
        define_method(name) do |*args|
          ivar = "@#{name}"

          if instance_variable_defined?(ivar)
            instance_variable_get(ivar)
          else
            instance_variable_set(ivar, instance_exec(block, *args, &block))
          end
        end
      else
        attr_reader name
      end

      helper_method name
    end
  end
end

3-instance_variables.rb

module ApplicationHelper
  def method_missing(name, *args, &block)
    ivar = :"@#{name}"

    return super unless instance_variable_defined?(ivar)
    instance_variable_get(ivar)
  end
end

class CommentsController < ApplicationController
  load_and_authorize_resource :prepend => true, :through => :commentable

  expose(:comment) { commentable.model.comments.find(params[:id]) }

  def create
    comment = commentable.comments.build(comment_params)
    authorize!(:create, comment)

    if comment.save
      flash[:notice] = 'Comment successfully added!'
      redirect_to commentable
    else
      instance_variable_set(:"@#{comment.commentable_type.underscore}", commentable)
      flash.now[:alert] = 'Comment could not be added!'
      render "#{comment.commentable_type.tableize}/show"
    end
  end
end

4-locals_hash.rb

class CommentsController < ApplicationController
  load_and_authorize_resource :prepend => true, :through => :commentable

  expose(:comment) { commentable.model.comments.find(params[:id]) }

  def create
    comment = commentable.comments.build(comment_params)
    authorize!(:create, comment)

    if comment.save
      flash[:notice] = 'Comment successfully added!'
      redirect_to commentable
    else
      flash.now[:alert] = 'Comment could not be added!'
      render "#{comment.commentable_type.tableize}/show", :locals => { comment.commentable_type.underscore.to_sym => commentable }
    end
  end
end

5-instance_level_expose.rb

# Pseudo-code – this is what I'd like to be able to do.
class CommentsController < ApplicationController
  load_and_authorize_resource :prepend => true, :through => :commentable

  expose(:comment) { commentable.model.comments.find(params[:id]) }

  def create
    comment = commentable.comments.build(comment_params)
    authorize!(:create, comment)

    if comment.save
      flash[:notice] = 'Comment successfully added!'
      redirect_to commentable
    else
      expose(comment.commentable_type.underscore, commentable)
      flash.now[:alert] = 'Comment could not be added!'
      render "#{comment.commentable_type.tableize}/show"
    end
  end
end

So the problem is that you are rendering the show view for a resource, e.g. contact, and understandably using the name of the resource in the view code rather than a generic term.

I think :locals is the most correct approach in this situation. I probably would have structured my controllers differently to avoid the situation. For example by having a controller for each commentable resource that inherits from a base commentable controller but has its own expose method for the resource. So your route would be /contacts/23/comments/ and a ContactCommentsController < CommentsController. Because I'd prefer to load the resource model directly then build the comment on it rather than the other way around. Also I often need to do more to render the "show" page of a resource than just loading a single model. Can be nicely solved with AJAX too.

Sorry probably not very helpful!