MONGOID-4889 Optimize batch assignment of embedded documents

[jamis]

When associating multiple embedded documents in a single assignment, the existing code was inefficiently looping over all records repeatedly, resulting in a significant performance impact. This PR attempts to minimize the number of loops needed to accomplish the same thing, while keeping the ordering of callbacks the same and otherwise attempting to not break backward compatibility.

To test the performance of this PR, the following benchmark (from MONGOID-4889) was used:

class Foo include Mongoid::Document embeds_many :bars end class Bar include Mongoid::Document embedded_in :foo end require 'benchmark' array_1k = Array.new(1000) { Bar.new } array_2k = Array.new(2000) { Bar.new } array_3k = Array.new(3000) { Bar.new } array_4k = Array.new(4000) { Bar.new } array_5k = Array.new(5000) { Bar.new } Benchmark.bm do |x| x.report('1k') { Foo.new.bars = array_1k } x.report('2k') { Foo.new.bars = array_2k } x.report('3k') { Foo.new.bars = array_3k } x.report('4k') { Foo.new.bars = array_4k } x.report('5k') { Foo.new.bars = array_5k } end

With the previous implementation, the timings were abyssmal:

 user system total real 1k 0.721604 0.002515 0.724119 ( 0.724148) 2k 2.834939 0.008708 2.843647 ( 2.843895) 3k 6.383648 0.017409 6.401057 ( 6.401207) 4k 11.313247 0.035579 11.348826 ( 11.349941) 5k 17.895191 0.096859 17.992050 ( 18.048693) 

Things look much better with the optimized implementation:

 user system total real 1k 0.031262 0.000538 0.031800 ( 0.031798) 2k 0.041094 0.000543 0.041637 ( 0.041638) 3k 0.058909 0.000606 0.059515 ( 0.059517) 4k 0.090872 0.000752 0.091624 ( 0.091641) 5k 0.096289 0.001358 0.097647 ( 0.097666) 

[johnnyshields]

Awesome, great work ❤️