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Type: Improvement
Resolution: Fixed
Priority: Major - P3
Fix Version/s: 7.2.0.rc1
Affects Version/s: 7.0.8
Component/s: None
Labels:
None
Environment:
Ubuntu 18, Phusion Passenger, Ruby 2.5.8 Mongo Atlas with Global Write Enabled

Case:

Release Notes:
When Mongoid reloads models it will use the shard key declared for the model, if one exists, in addition to the id field in the find query.

Given a Global Write enabled Mongo Atlas cluster with the following compound shard keys:

{ location: 1, token: 1 }

The reload mechanism for my document takes around 200 ms. This severely affects the performance of my application since all of the invocations of delete_all/destroy_all seems to utilize the reload mechanism on the document.

It also appears that the "identity" of the document is not compatible with Global Write clusters, since it only utilizes the document's ID

Here's some code to demonstrate the performance issues:

mc = nil
Benchmark.bm(100) do |bm|
  bm.report("Initialize Mongo connection and get Mongo Cart") do 
    mc = MongoCart.where(location: "CA", token: "4d8e7b38d7ac73ba68cf6401299b918f").first
  end 

  bm.report("Re-fetch Mongo Cart") do 
    mc = MongoCart.where(location: "CA", token: "4d8e7b38d7ac73ba68cf6401299b918f").first
  end 

  bm.report("Reload Mongo Cart") do 
    mc.reload
  end

  bm.report("Get Mongo Cart by Token") do 
    mc = MongoCart.find_by(token: "4d8e7b38d7ac73ba68cf6401299b918f")
  end

  bm.report("Get Mongo Cart by ID") do 
    mc = MongoCart.find(mc._id)
  end

  bm.report("Get Mongo Cart by Location + ID") do 
    mc = MongoCart.where(location: "CA", _id: mc._id).first
  end
end