Try to repeat error for mongoid7

one_app.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'mongoid', '~> 7.0'
  gem 'mongoid_relations_dirty_tracking', github: 'tablecheck/mongoid_relations_dirty_tracking'
  gem 'pry-byebug'
end

Mongoid.configure do |config|
  config.clients.default = {
    hosts: ['mongo:27017'],
    database: 'test_db',
  }

  config.log_level = :warn
end

class Article
  include Mongoid::Document
  include Mongoid::RelationsDirtyTracking

  embeds_many :comments, as: :commentable, class_name: 'Comment'

  field :title
end

class Comment
  include Mongoid::Document

  embedded_in :commentable, polymorphic: true

  field :text
end

article = Article.new(title: "MainArticle")
article.comments << Comment.new(text: "Some_text1")
article.comments << Comment.new(text: "Some_text2")
article.comments << Comment.new(text: "Some_text3")
article.save!

article.comments.first.assign_attributes(text: "Other_text")
article.save!

Fixed