My current best approach for creating polymorphic child records based on the best bits from all other approaches.

This example - creating polymorphic Comments table that can be integrated easily into any model.

polymorphic-comments.gif

Step 1. Add polymorphic comments to the application

console

rails g migration create_comments content:text commentable:references{polymorphic}

app/models/comment.rb

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
  validates :content, presence: true
  def to_s
    content
  end
end 

app/controllers/comments_controller.rb

class CommentsController < ApplicationController
  before_action :set_commentable

  def new
    @comment = Comment.new
  end

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

  def destroy
    @comment = Comment.find(params[:id])
    if @comment.destroy
      redirect_to @commentable, notice: "Comment deleted"
    else
      redirect_to @commentable, alert: "Something went wrong"
    end
  end

  private

  def comment_params
    params.require(:comment).permit(:content)
  end

  def set_commentable
    if params[:user_id].present?
      @commentable = User.find_by_id(params[:user_id])
    elsif params[:lesson_id].present?
      @commentable = Lesson.find_by_id(params[:lesson_id])
    end
  end
end

app/views/comments/_form.html.erb

<%= form_for [@commentable, @comment] do |f| %>
  <% if @comment.errors.any? %>
    <% @comment.errors.each do |error| %>
      <%= error.full_message %>
    <% end %>
  <% end %>

  <%= f.label :content %>
  <%= f.text_area :content %>
  <%= f.submit %>
<% end %>

app/views/comments/_index.html.erb

Comments:
<%= @commentable.comments.count %>
<% commentable.comments.each do |comment| %>
  <%= comment.created_at.strftime('%d-%m-%Y %H:%m') %>
  <%= link_to "Destroy", [@commentable, comment], method: :delete %>
  <%= simple_format(comment.content) %>
  <br>
<% end %>

app/views/comments/new.html.erb

New Comment for:
<%= link_to @commentable, @commentable %>
<%= render 'comments/form' %>

routes.rb

  resources :lessons do
    resources :comments, only: [:new, :create, :destroy]
  end

Step 2. Add polymorphic comments to a model. For example lesson

app/controllers/lessons_controller.rb

  def show
    @commentable = @lesson
    @comment = Comment.new
  end

app/models/lesson.rb

   has_many :comments, as: :commentable, dependent: :destroy

app/views/lessons/show.html.erb - with a link to create a comment

<%= link_to "New Comment", new_lesson_comment_path(@commentable, @comment) %>
<%= render partial: "comments/index", locals: {commentable: @commentable} %>

app/views/lessons/show.html.erb - alternative - with a comments form in the view

<%= render template: "comments/new" %>
<%= render partial: "comments/index", locals: {commentable: @commentable} %>

Notes

In comments_controller you have to update the action set_commentable with each model that you want to make commentable. This is better than creating a separate comments_controller for each commentable model.

To add polymorphic to one more model, repeat step 2 by replacing the word lesson with whatever model you want.

Bonus: Save which user created a comment

  1. rails g migration add_user_references_to_comments user:references
    
  2. To save current_user who created a comment, in comments_controller create action above line if @comment.save add the line
    @comment.user_id = current_user.id
    

Using simple_form

<%= simple_form_for [@commentable, @comment] do |f| %>
  <%= f.error_notification %>
  <%= f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present? %>

  <%= f.input :content, label: false, required: true %>
  <%= f.button :submit %>
<% end %>