PERF: Restore memoization when preloading associations.

[tgxworld]

Extracted from #30550

Benchmark Script

require 'active_record'
require 'benchmark/ips'
require 'ruby-prof'
require 'memory_profiler'
require 'byebug'

ActiveRecord::Base.establish_connection(ENV.fetch('DATABASE_URL'))
ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.integer :topic_id
    t.timestamps null: false
  end

  create_table :topics, force: true do |t|
    t.string :title
    t.timestamps null: false
  end
end

attributes = {
  name: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
  email: 'foobar@email.com'
}

class Topic < ActiveRecord::Base
  has_many :users
end

class User < ActiveRecord::Base
  belongs_to :topic
end

100.times do
  User.create!(attributes)
end

users = User.first(50)

Topic.create!(title: 'This is a topic', users: users)

Benchmark.ips do |x|
  x.config(time: 10, warmup: 5)

  x.report("preload") do
    User.includes(:topic).all.to_a
  end
end

Before

Calculating -------------------------------------
             preload    26.000  i/100ms
-------------------------------------------------
             preload    265.347  (± 3.0%) i/s -      2.652k

After

Calculating -------------------------------------
             preload    39.000  i/100ms
-------------------------------------------------
             preload    406.053  (± 1.7%) i/s -      4.095k

[tgxworld]

r? @kamipo