Given...

class Account
include Mongoid::Document

field :name, :type => String

embeds_many :memberships
end

class Membership
include Mongoid::Document
include Mongoid::Timestamps

field :admin, :type => Boolean, :default => false

embedded_in :account, :inverse_of => :memberships
end

And I do...

a = Account.new(:name => "Test")
a.memberships << Membership.new
a.save

Timestamps will fail...

a.memberships.first.created_at # nil
Account.first.memberships.first.created_at # nil

However if I...

a.memberships.create

Then it works...

a.memberships.last.created_at # Nov 3...
Account.first.memberships.last.created_at # Nov 3..

And on another note if I...

a = Account.new(:name => "Test")
a.memberships.build
a.save

The membership will not be persisted at all...

a.memberships.first # Membership
Account.first.memberships.first # nil