ActiveRecord::AttributeMethods::Dirtyモジュールで定義している以下メソッドの挙動をまとめました。

*_before_last_save
*_in_database
saved_change_to_*
will_save_change_to_*?
saved_change_to_*?
*_change_to_be_saved
saved_changes?
saved_changes
has_changes_to_save?
changes_to_save
changed_attribute_names_to_save
attributes_in_database

※順番はテキトーです

以下のコードを1行ずつ実行し、各時点の dirty method の返り値を表にまとめました。

o = Obj.create(foo: "V1")
o.foo = "V2"
o.save!
o.foo = "V3"

一覧

active_record 6.0.2.2

Create record

  o = Obj.create(foo: "V1")

method	before_save	after_save	created
`foo`	`"V1"`	`"V1"`	`"V1"`
`foo_before_last_save`	`nil`	`nil`	`nil`
`foo_in_database`	`nil`	`"V1"`	`"V1"`
`saved_change_to_foo`	`nil`	`[nil, "V1"]`	`[nil, "V1"]`
`will_save_change_to_foo?`	`true`	`false`	`false`
`saved_change_to_foo?`	`false`	`true`	`true`
`foo_change_to_be_saved`	`[nil, "V1"]`	`nil`	`nil`
`saved_changes?`	`false`	`true`	`true`
`saved_changes`	`{}`	`{"id"=>[nil, 1], "foo"=>[nil, "V1"]}`	`{"id"=>[nil, 1], "foo"=>[nil, "V1"]}`
`has_changes_to_save?`	`true`	`false`	`false`
`changes_to_save`	`{"foo"=>[nil, "V1"]}`	`{}`	`{}`
`changed_attribute_names_to_save`	`["foo"]`	`[]`	`[]`
`attributes_in_database`	`{"foo"=>nil}`	`{}`	`{}`

Change record

  o.foo = "V2"

method	changed
`foo`	`"V2"`
`foo_before_last_save`	`nil`
`foo_in_database`	`"V1"`
`saved_change_to_foo`	`[nil, "V1"]`
`will_save_change_to_foo?`	`true`
`saved_change_to_foo?`	`true`
`foo_change_to_be_saved`	`["V1", "V2"]`
`saved_changes?`	`true`
`saved_changes`	`{"id"=>[nil, 1], "foo"=>[nil, "V1"]}`
`has_changes_to_save?`	`true`
`changes_to_save`	`{"foo"=>["V1", "V2"]}`
`changed_attribute_names_to_save`	`["foo"]`
`attributes_in_database`	`{"foo"=>"V1"}`

Save record

  o.save!

method	before_save	after_save	save finished
`foo`	`"V2"`	`"V2"`	`"V2"`
`foo_before_last_save`	`nil`	`"V1"`	`"V1"`
`foo_in_database`	`"V1"`	`"V2"`	`"V2"`
`saved_change_to_foo`	`[nil, "V1"]`	`["V1", "V2"]`	`["V1", "V2"]`
`will_save_change_to_foo?`	`true`	`false`	`false`
`saved_change_to_foo?`	`true`	`true`	`true`
`foo_change_to_be_saved`	`["V1", "V2"]`	`nil`	`nil`
`saved_changes?`	`true`	`true`	`true`
`saved_changes`	`{"id"=>[nil, 1], "foo"=>[nil, "V1"]}`	`{"foo"=>["V1", "V2"]}`	`{"foo"=>["V1", "V2"]}`
`has_changes_to_save?`	`true`	`false`	`false`
`changes_to_save`	`{"foo"=>["V1", "V2"]}`	`{}`	`{}`
`changed_attribute_names_to_save`	`["foo"]`	`[]`	`[]`
`attributes_in_database`	`{"foo"=>"V1"}`	`{}`	`{}`

Change record

  o.foo = "V3"

method	changed
`foo`	`"V3"`
`foo_before_last_save`	`"V1"`
`foo_in_database`	`"V2"`
`saved_change_to_foo`	`["V1", "V2"]`
`will_save_change_to_foo?`	`true`
`saved_change_to_foo?`	`true`
`foo_change_to_be_saved`	`["V2", "V3"]`
`saved_changes?`	`true`
`saved_changes`	`{"foo"=>["V1", "V2"]}`
`has_changes_to_save?`	`true`
`changes_to_save`	`{"foo"=>["V2", "V3"]}`
`changed_attribute_names_to_save`	`["foo"]`
`attributes_in_database`	`{"foo"=>"V2"}`

検証用に使用したスクリプト

# frozen_string_literal: true

require "bundler/inline"

ACTIVERECORD_VERSION = '6.0.2.2'

$stdout = open(File::NULL, "w") # Suppress migration log

gemfile(true) do
  source "https://rubygems.org"

  gem "activerecord", ACTIVERECORD_VERSION, require: "active_record"
  gem "sqlite3"
end

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

ActiveRecord::Schema.define do
  create_table :objs do |t|
    t.string :foo, null: false
  end
end

$stdout = STDOUT

class Obj < ActiveRecord::Base
  METHODS = %i(
    foo
    foo_before_last_save
    foo_in_database
    saved_change_to_foo
    will_save_change_to_foo?
    saved_change_to_foo?
    foo_change_to_be_saved
    saved_changes?
    saved_changes
    has_changes_to_save?
    changes_to_save
    changed_attribute_names_to_save
    attributes_in_database
  ).freeze

  Captured = Struct.new(:context, :results)

  after_initialize :clear

  before_save do
    capture("before_save")
  end

  after_save do
    capture("after_save")
  end

  def capture(context)
    results = METHODS.map do |method|
      [method, public_send(method).inspect]
    end.to_h

    @captured << Captured.new(context, results)
  end

  def clear
    @captured = []
  end

  def inspect
    table = []

    table << "| method |" + @captured.map { |cap| " #{cap.context} " }.join("|") + "|"
    table << "|---" + ("|---" * @captured.length) + "|"

    METHODS.each do |method|
      table << "| `#{method}` |" + @captured.map { |cap| " `#{cap.results[method]}` " }.join("|") + "|"
    end

    table.join("\n") + "\n"
  end
end

BINDING = binding

def action(desc, code)
  puts <<~END
    ### #{desc}

    ```ruby
      #{code}
    ```

  END

  BINDING.eval(code)
end

puts "## active_record #{ACTIVERECORD_VERSION}\n\n"

o = action("Create record", 'o = Obj.create(foo: "V1")')
o.capture("created")
p o
o.clear

action("Change record", 'o.foo = "V2"')
o.capture("changed")
p o
o.clear

action("Save record", 'o.save!')
o.capture("save finished")
p o
o.clear

action("Change record", 'o.foo = "V3"')
o.capture("changed")
p o
o.clear

ActiveRecord6 の dirty attribute methods （will_save_change_to_* とか）の挙動一覧

一覧

active_record 6.0.2.2

Create record

Change record

Save record

Change record

検証用に使用したスクリプト

参考