create! と違い create は例外を起こさない
返り値が model class の instance になるので、 persisted? ( 永続化されたかどうか？ ) を聞くと良さそうだ。
SQLレベルで失敗した場合は例外扱いになり、返り値は得られないようだ。


# -------------------------
# 作成成功ケース
# -------------------------

succeeded_created = User.create(name: "yy")
# (1.0ms)  BEGIN
#   User Exists (1.4ms)  SELECT  1 AS one FROM `users` WHERE `users`.`name` = BINARY 'yy' LIMIT 1
#   User Create (1.4ms)  INSERT INTO `users` (`name`, `created_at`) VALUES ('yy', '2020-02-13 23:42:17')
#    (4.9ms)  COMMIT

succeeded_created.persisted?
# => true

succeeded_created.valid?
#  User Exists (1.0ms)  SELECT  1 AS one FROM `users` WHERE `users`.`name` = BINARY 'yy' AND `users`.`id` != 83 LIMIT 1
# => true

succeeded_created.invalid?
#  User Exists (2.2ms)  SELECT  1 AS one FROM `users` WHERE `users`.`name` = BINARY 'yy' AND `users`.`id` != 83 LIMIT 1
# => false

succeeded_created.errors
#=> #<ActiveModel::Errors:0x000056396e61bc70
# @base=#<User:0x000056396e816480 id: 83, name: "yy", created_at: Thu, 13 Feb 2020 23:45:53 UTC +00:00>,
# @details={},
# @messages={}>

succeeded_created.errors.present?
#=> false


# -------------------------
# バリデーションによる作成失敗ケース
# -------------------------


failed_created = User.create(name: "yy")
#    (0.6ms)  BEGIN
#  User Exists (1.2ms)  SELECT  1 AS one FROM `users` WHERE `users`.`name` = BINARY 'yy' LIMIT 1
#   (1.0ms)  ROLLBACK

failed_created.persisted?
# => false


failed_created.errors
# => #<ActiveModel::Errors:0x0000563973bd4888
# @base=#<User:0x0000563973bd9d10 id: nil, name: "yy", created_at: nil>,
# @details={:name=>[{:error=>:taken, :value=>"yy"}]},
# @messages={:name=>["はすでに存在します"]}>

failed_created.errors.present?
# => true

failed_created.valid?
 #  User Exists (1.2ms)  SELECT  1 AS one FROM `users` WHERE `users`.`name` = BINARY 'yy' LIMIT 1
=> false

failed_created.invalid?
#   User Exists (1.7ms)  SELECT  1 AS one FROM `users` WHERE `users`.`name` = BINARY 'yy' LIMIT 1
=> true


# -----------------------------
# SQL エラーが発生したケース
# -----------------------------

sql_failed_created = User.create(name: "yy")
#   (0.8ms)  BEGIN
 # User Create (6.1ms)  INSERT INTO `users` (`name`, `created_at`) VALUES ('yy', '2020-02-13 23:55:11')
#   (5.7ms)  ROLLBACK
#ActiveRecord::RecordNotUnique: Mysql2::Error: Duplicate entry 'yy' for key 'index_users_on_name': INSERT INTO `users` (`name`, `created_at`) VALUES ('yy', '2020-02-13 23:55:11')
# 
# Caused by Mysql2::Error: Duplicate entry 'yy' for key 'index_users_on_name'

sql_failed_created.persisted?
# NoMethodError: undefined method `persisted?' for nil:NilClass

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