Revert "Revert "Reduce allocations when running AR callbacks.""

This reverts commit bdc1d329d4eea823d07cf010064bd19c07099ff3.

Before:
Calculating -------------------------------------
                        22.000  i/100ms
-------------------------------------------------
                        229.700  (± 0.4%) i/s -      1.166k
Total Allocated Object: 9939

After:
Calculating -------------------------------------
                        24.000  i/100ms
-------------------------------------------------
                        246.443  (± 0.8%) i/s -      1.248k
Total Allocated Object: 7939

```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # gem 'rails', github: 'rails/rails', ref: 'bdc1d329d4eea823d07cf010064bd19c07099ff3'
  gem 'rails', github: 'rails/rails', ref: 'd2876141d08341ec67cf6a11a073d1acfb920de7'
  gem 'arel', github: 'rails/arel'
  gem 'sqlite3'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('sqlite3::memory:')

ActiveRecord::Migration.verbose = false

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

class User < ActiveRecord::Base
  default_scope { where(admin: true) }
end

admin = true

1000.times do
  attributes = {
    name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
    email: "foobar@email.com",
    admin: admin
  }

  User.create!(attributes)

  admin = !admin
end

GC.disable

Benchmark.ips(5, 3) do |x|
  x.report { User.all.to_a }
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.all.to_a
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

1 files changed, 4 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 8a014e682e..b82488a59c 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -303,7 +303,7 @@ module ActiveRecord
       assign_attributes(attributes) if attributes
 
       yield self if block_given?
-      run_callbacks :initialize
+      _run_initialize_callbacks
     end
 
     # Initialize an empty model object from +coder+. +coder+ should be
@@ -330,8 +330,8 @@ module ActiveRecord
 
       self.class.define_attribute_methods
 
-      run_callbacks :find
-      run_callbacks :initialize
+      _run_find_callbacks
+      _run_initialize_callbacks
 
       self
     end
@@ -367,7 +367,7 @@ module ActiveRecord
       @attributes = @attributes.dup
       @attributes.reset(self.class.primary_key)
 
-      run_callbacks(:initialize)
+      _run_initialize_callbacks
 
       @new_record  = true
       @destroyed   = false