Consider the following:

class User
  include Mongoid::Document

  embeds_many :accounts

end

class Account
  include Mongoid::Document

  embedded_in :user

  field :name, :type String
  validates_presence_of :name

end

u = User.find('xxx')
u.accounts.to_a.count # 0
a = Account.new(:user => u)
a.valid? # false
u.accounts.to_a.count # 1

This creates all sorts of headaches when rendering a view which includes the Account instance being altered AND the collection of Account instances (as the new Account object appears in u.accounts).

I'm pretty sure this doesn't happen with AR as u.accounts would execute a fetch from the DB.

My only workaround is to run u.reload before retrieving the collection of items.