Avoid generating full changes hash on every save

[eugeneius]

changed_attribute_names_to_save is called in keys_for_partial_write, which is called on every save when partial writes are enabled.

We can avoid generating the full changes hash by asking the mutation tracker for just the names of the changed attributes. At minimum this saves one array allocation per attribute, but will also avoid calling Attribute#original_value which is expensive for serialized attributes.

r? @sgrif

[eugeneius]

Benchmarks:

Simple case: model with no attributes

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", path: "."
  gem "sqlite3"
  gem "benchmark-ips"
end

require "active_record"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end
end

class Post < ActiveRecord::Base
end

post = Post.new

Benchmark.ips do |x|
  x.report("mutations_from_database.changed_attribute_names") { post.changed_attribute_names_to_save }
  x.report("changes_to_save.keys") { post.changes_to_save.keys }
end

Warming up --------------------------------------
mutations_from_database.changed_attribute_names
                        39.860k i/100ms
changes_to_save.keys    25.917k i/100ms
Calculating -------------------------------------
mutations_from_database.changed_attribute_names
                        477.041k (± 3.6%) i/s -      2.392M in   5.019760s
changes_to_save.keys    286.142k (± 4.2%) i/s -      1.451M in   5.080372s

Complex case: model with a large serialized attribute

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", path: "."
  gem "sqlite3"
  gem "benchmark-ips"
end

require "active_record"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.string :content
  end
end

class Post < ActiveRecord::Base
  serialize :content
end

post = Post.new(content: [{ foo: "bar" }] * 100)

Benchmark.ips do |x|
  x.report("mutations_from_database.changed_attribute_names") { post.changed_attribute_names_to_save }
  x.report("changes_to_save.keys") { post.changes_to_save.keys }
end

Warming up --------------------------------------
mutations_from_database.changed_attribute_names
                        21.255k i/100ms
changes_to_save.keys     3.398k i/100ms
Calculating -------------------------------------
mutations_from_database.changed_attribute_names
                        231.289k (± 3.4%) i/s -      1.169M in   5.060643s
changes_to_save.keys     35.805k (± 2.3%) i/s -    180.094k in   5.032551s

[kamipo]

This improvement was accidentally lost in the revert commit 5fcbdcf.