What is the Rails way to work with polymorphic associations?

I have few models in my Rails application, which are:

1. User
2. Photo
3. Album
4. Comment

I need to make comments belog to either to Photo or Album, and obviously always belong to User. I'm going to use polymorphic associations for that.

# models/comment.rbclass Comment < ActiveRecord::Base  belongs_to :user  belongs_to :commentable, :polymorphic => trueend

The question is, what is the Rails way to describe #create action for the new comment. I see two options for that.

1. Describe the comment creation in each controller

But ths is not a DRY solution. I can make one common partial view for displaying and creating comments but I will have to repeat myself writing comments logic for each controller. So It doesn't work

2. Create new CommentsController

This is the right way I guess, but as I aware:

To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface

Like this:

# schema.rb  create_table "comments", force: :cascade do |t|    t.text     "body"    t.integer  "user_id"    t.integer  "commentable_id"    t.string   "commentable_type"    t.datetime "created_at",       null: false    t.datetime "updated_at",       null: false  end

So, when I will be writing pretty simple controller, which will be accepting requests from the remote form:

# controllers/comments_controller.rbclass CommentsController < ApplicationController  def new    @comment = Comment.new  end  def create    @commentable = ??? # How do I get commentable id and type?    if @comment.save(comment_params)      respond_to do |format|        format.js {render js: nil, status: :ok}      end    end  end  private  def comment_params    defaults = {:user_id => current_user.id,                 :commentable_id => @commentable.id,                 :commentable_type => @commentable.type}    params.require(:comment).permit(:body, :user_id, :commentable_id,                                     :commentable_type).merge(defaults)  endend

How will I get commentable_id and commetable_type? I guess, commentable_type might be a model name.

Also, what is the best way to make a form_for @comment from other views?