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ele é confrontado com esta fera maligna:

ActiveRecord::RecordNotFound in CommentsController#create
Couldn't find Comment with 'id'=
Line: @commentable = resource.singularize.classify.constantize.find(id)

atividades / índice

<%= link_to activity.user.name, activity.user %>
<%= render "activities/#{activity.trackable_type.underscore}/#{activity.action}", activity: activity %>
<%= render "comments/comments", comments: activity.comments %>
<%= render "comments/form", new_comment: Comment.new(commentable_id: activity.id, commentable_type: activity.class.model_name) %>

comments / _form

<%= form_for new_comment do |f| %>
  <%= f.text_area :content, rows: 4, class: 'form-control', placeholder: 'Enter Comment' %>
  <%= button_tag(type: 'submit', class: "btn") do %>
    <span class="glyphicon glyphicon-plus"></span> Comment
  <% end %>
<% end %>

activities_controller

def index
  @activities = Activity.order("created_at desc").where(user_id: current_user.following_ids)
  @commentable = @activity
  @comment = Comment.new
end

O erro pode ser encontrado aqui:

class CommentsController < ApplicationController
  before_action :load_commentable
  before_action :set_comment, only: [:show, :edit, :update, :destroy, :like]
  before_action :logged_in_user, only: [:create, :destroy]

    def index
        @comments = @commentable.comments
    end

    def new
        @comment = @commentable.comments.new
    end

    def create
        @comment = @commentable.comments.new(comment_params)
        if @comment.save
            redirect_to @commentable, notice: "comment created."
        else
            render :new
        end
    end

    def edit
        @comment = current_user.comments.find(params[:id])
    end

    def update
        @comment = current_user.comments.find(params[:id])
        if @comment.update_attributes(comment_params)
            redirect_to @commentable, notice: "Comment was updated."
        else
            render :edit
        end
    end

    def destroy
        @comment = current_user.comments.find(params[:id])
        @comment.destroy
        redirect_to @commentable, notice: "comment destroyed."
    end

  def like
    @comment = Comment.find(params[:id])
    @comment_like = current_user.comment_likes.build(comment: @comment)
    if @comment_like.save
            @comment.increment!(:likes)
        flash[:success] = 'Thanks for liking!'
    else
        flash[:error] = 'Two many likes'
      end  
        redirect_to(:back)
  end

private
  def set_comment
    @comment = Comment.find(params[:id])
  end

    def load_commentable
        resource, id = request.path.split('/')[1, 2]
        @commentable = resource.singularize.classify.constantize.find(id) #Here it is!
    end

    def comment_params
        params[:comment][:user_id] = current_user.id
        params.require(:comment).permit(:content, :commentable, :user_id, :like)
    end
end

O erro surgiu da resposta aqui:Como adicionar comentários polimórficos ao feed?

development.log

Started POST "/comments" for ::1 at 2015-04-23 20:12:14 -0400
Processing by CommentsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"hU1lxg2BMqSyBo8j2SGiEB4wZ3ez5kz/E64mp6ssbwBnh+DddyTtNQxY+IYCluHHvs2wIBxrtD5hQVA5sGtXBg==", "comment"=>{"content"=>"test"}, "button"=>""}
  [1m[35mUser Load (0.2ms)[0m  SELECT  "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1  [["id", 2]]
  [1m[36mHabit Load (0.1ms)[0m  [1mSELECT "habits".* FROM "habits" WHERE "habits"."user_id" = ?[0m  [["user_id", 2]]
  [1m[35mHabit Load (2.5ms)[0m  SELECT "habits".* FROM "habits"
  [1m[36mActsAsTaggableOn::Tag Load (0.3ms)[0m  [1mSELECT "tags".* FROM "tags" WHERE (LOWER(name) = LOWER('ingrain'))[0m
  [1m[35mComment Load (0.3ms)[0m  SELECT  "comments".* FROM "comments" WHERE "comments"."id" = ? LIMIT 1  [["id", nil]]
Completed 404 Not Found in 16ms

ActiveRecord::RecordNotFound (Couldn't find Comment with 'id'=):
  app/controllers/comments_controller.rb:61:in `load_commentable'

Muito obrigado!