Validates fields and embedded documents that were not changed

According to documentation Mongoid should not validate fields that were not changed.

> Mongoid behaves slightly different to Active Record when using #valid? on already persisted data. Active Record's #valid? will run all validations whereas Mongoid's #valid? will only run validations on fields where data has changed as an optimization.
> (http://mongoid.org/en/mongoid/docs/validation.html)

Here's a script that proves otherwise:

#!/usr/bin/env ruby

require "mongoid"

Mongoid.configure do |config|

  config.connect_to "mongoid_test"

end

class A

  include Mongoid::Document

  field :name, type: String

  validates :name, presence: true

end

A.delete_all

a = A.new

a.save! validate: false

a = A.first

puts "Changed: #{a.changed?}, #{a.name_changed?}"

puts "Valid: #{a.valid?}"

a.errors.full_messages.each do |m|

  puts m

end

Output:
> Changed: false, false
> Valid: false
> Name can't be blank

But frankly, I don't care about ordinary fields. I was expecting Mongoid to skip validation of embedded documents if they were not changed. This sounds like an obvious optimization and in various forms has been mentioned in different issues (like MONGOID-723). I have a document with 1.5k embedded documents. Running valid? on that parent document takes 50s on my production server. My users can't do anything on that document in my webapp.

If on the other hand I were to pass validate: false to the embeds_many definition, none of the newly added children would be validated.

Here's another example script, this time with embedded children:

#!/usr/bin/env ruby

require "mongoid"

Mongoid.configure do |config|

  config.connect_to "mongoid_test"

end

class B

  include Mongoid::Document

  embedded_in :a

  field :name, type: String

  validates :name, presence: true

end

class A

  include Mongoid::Document

  embeds_many :b

end

A.delete_all

a = A.new

5.times {a.b.build}

a.save! validate: false

a = A.first

puts "Changed: #{a.changed?}"

puts "B size: #{a.b.size}"

puts "Valid: #{a.valid?}"

a.errors.full_messages.each do |m|

  puts m

end

Output:
> Changed: false
> B size: 5
> Valid: false
> B is invalid