predictive_load

Observes Active Record collections and notifies when a member loads an association. This allows for:

• automatically preloading the association in a single query for all members of that collection.
• N+1 detection logging

require 'predictive_load'
require 'predictive_load/active_record_collection_observation'
ActiveRecord::Base.include(PredictiveLoad::ActiveRecordCollectionObservation)

require 'predictive_load/loader'

ActiveRecord::Relation.collection_observer = PredictiveLoad::Loader

Ticket.all.each do |ticket| 
  ticket.requester.identities.each { |identity| identity.account }
end

Produces:

  SELECT `tickets`.* FROM `tickets`
  SELECT `requesters`.* FROM `requesters` WHERE `requesters`.`id` IN (2, 7, 12, 32, 37)
  SELECT `identities`.* FROM `identities` WHERE `identities`.`requester_id` IN (2, 7, 12, 32, 37)
  SELECT `accounts`.* FROM `accounts` WHERE `accounts`.`id` IN (1, 2, 3)

Some things cannot be preloaded, use predictive_load: false

has_many :foos, predictive_load: false

The library can be instrumented by providing a callback, to be invoked every time automatic preloading happens. The callback must be a callable that receives two arguments:

• The record (instance) on which the queries that triggered automatic preloading are being performed, in the form of some association call.
• The association object, which can be inspected to check the type and name of the association.

For example, the callback could be used to emit some metrics:

require "active_support/core_ext/string"

PredictiveLoad.callback = -> (record, association) do
  METRICS_CLIENT.increment_counter(
    "active_record.automatic_preloads",
    tags: [
      "model:#{record.class.name.underscore}",
      "association:#{association.reflection.name}"
    ]
  )
end

• Calling association#size will trigger an N+1 on SELECT COUNT(*). Work around by calling #length, loading all records.
• Calling first / last will trigger an N+1.