require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'mongoid'
end

Tempfile.open(%w(config .yml)) do |f|
  uri = 'mongodb://mongo/test'

  f.write(<<~YAML)
development:
  clients:
    default:
      uri: #{uri}
  YAML
  f.flush

  Mongoid.load!(f.path, 'development')
  Mongoid::Clients.clients.each do |_, client|
    client.logger.level = Logger::ERROR
  end
end

class Parent
  include Mongoid::Document

  embeds_one :embedded_child
end

class EmbeddedChild
  include Mongoid::Document

  embedded_in :parent, polymorphic: true
end

EmbeddedChild.create!(parent: Parent.create!)

__END__
Mongoid::Errors::InvalidPath:
message:
  Having a root path assigned for EmbeddedChild is invalid.
summary:
  Mongoid has two different path objects for determining the location of a document in the database, Root and Embedded. This error is raised when an embedded document somehow gets a root path assigned.
resolution:
  Most likely your embedded model, EmbeddedChild is also referenced via a has_many from a root document in another collection. Double check the association definitions and fix any instances where embedded documents are improperly referenced from other collections.
from /usr/local/bundle/gems/mongoid-7.1.2/lib/mongoid/atomic/paths/root.rb:36:in `insert_modifier'