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

Automatic preloading

“by 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:

“l 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)

“

Disabling preload

Some things cannot be preloaded, use predictive_load: false

“as_many :foos, predictive_load: false

“

Instrumentation

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:

“by require “active_support/core_ext/string”

PredictiveLoad.callback = -> (record, association) do METRICS_CLIENT.increment_counter( “active_record.automatic_preloads”, tags: [model:#recordrecord.classrecord.class.namerecord.class.name.underscore, association:#associationassociation.reflectionassociation.reflection.name] ) end

“

Known limitations:

• 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.