How to add a query comment to all queries?

[KieranP]

I am trying to debug some performance issues with our MongoDB Atlas database.

The performance advisor is good at suggesting indexes, but it doesn't have context to show us where the query was run from.

After a bit of research, it does appear that we can add a comment to queries on a individual basis:

User.collection.where(
  { some: "criteria"},
  { comment: "Something here" },
)

What I am looking for is a way to set this for ALL queries made during a Rails request. e.g.

class ApplicationController
  around_action do
    Mongoid.default_comment = request.path
    yield
    Mongoid.default_comment = nil
  end
end

The idea being that all queries will be commented with the URL path they were executed on, helping us pinpoint where the query ran so that we can change the underlying query when indexes can't help.

Does Mongoid have such a thing?
If not, would such a thing be possible to achieve?
If so, would such a thing be considered for development?
If not, what would others recommend?

Thanks :-)

[johnnyshields]

This would be a useful feature to have. Should be done as a block Mongoid.with_default_comment { ... }

[alexbevi]

@KieranP this is a great idea! I've opened https://jira.mongodb.org/browse/MONGOID-5903 to capture this as a feature request. I'm sure you could accomplish something similar as follows - though this might not be the best approach (which is why I opened the feature request 😄)

# config/initializers/mongoid_callsite_comment.rb
module MongoidCallsiteComment
  def execute(connection, context:, options: {})    
    # Duplicate the spec so we don't mutate driver internals in place
    spec = @spec.dup
    
    loc = caller_locations.first
    spec[:comment] ||= loc.to_s if spec.is_a?(Hash)

    # Swap the spec for this execution only
    orig_spec = @spec
    @spec= spec
    super
  ensure
    @spec= orig_spec
  end  
end

begin
  require "mongo"
  Mongo::Operation::Executable.prepend(MongoidCallsiteComment)
rescue NameError
  # If driver loads late in your boot order, defer until it’s loaded.
  ActiveSupport.on_load(:mongoid) do
    Mongo::Operation::Executable.prepend(MongoidCallsiteComment)
  end
end

A quick test will produce results such as the following (assuming the slow query threshold is low enough to log the operation):

{"t":{"$date":"2025-10-22T15:22:11.963-04:00"},"s":"I","c":"COMMAND","id":51803,"ctx":"conn44","msg":"Slow query","attr":{"type":"command","isFromUserConnection":true,"ns":"admin.test","collectionType":"admin","command":{"find":"test","$db":"admin","comment":"/Users/alex/.rvm/gems/ruby-3.4.1/gems/mongo-2.21.3/lib/mongo/server/connection_pool.rb:722:in 'Mongo::Server::ConnectionPool#with_connection'","filter":{"foo":"bar"},"limit":1,"sort":{"baz":-1},"lsid":{"id":{"$uuid":"574ebc11-bfc9-403a-9785-6c4a4a9cf7cd"}}},"planSummary":"COLLSCAN","planningTimeMicros":210,"keysExamined":0,"docsExamined":1,"hasSortStage":true,"nBatches":1,"cursorExhausted":true,"numYields":0,"nreturned":0,"planCacheShapeHash":"EEE9F651","planCacheKey":"7701AF85","queryFramework":"classic","reslen":224,"locks":{"Global":{"acquireCount":{"r":1}}},"readConcern":{"level":"local","provenance":"implicitDefault"},"storage":{},"remote":"127.0.0.1:59008","protocol":"op_msg","queues":{"execution":{"admissions":2},"ingress":{"admissions":1}},"workingMillis":0,"durationMillis":0}}

If this is in a Rails app the caller info should be more reflective of where the operation originated (I just did this in IRB).