try_account --- 更新時にnilで保存できないようにしました。

account.rb

# bundle add bcrypt
# rails g scaffold Account email password_digestx
# rails db:migrate
class Account < ApplicationRecord
  has_secure_password validations: false
  validate do |record|
    record.errors.add(:password, :blank) unless record.password_digest.present?
  end
  validates :password, format: { with: /\A[a-z0-9]+\z/i }, length: { in: 8..30 }, on: :create, unless: -> { password_digest.blank? }
  validates :password, format: { with: /\A[a-z0-9]+\z/i }, length: { in: 8..30 }, on: :update, allow_nil: true
end

account_test.rb

require "test_helper"

class AccountTest < ActiveSupport::TestCase
  test "生成" do
    account = Account.new
    assert account.invalid?
    assert_equal ["Password can't be blank"], account.errors.full_messages
    account.password = "!!!"
    assert account.invalid?
    assert_equal ["Password is invalid", "Password is too short (minimum is 8 characters)"], account.errors.full_messages
    account.password = "12345678"
    assert account.valid?
    assert account.save!
  end

  test "更新" do
    assert account = Account.create!(password: "12345678")
    account = Account.find(account.id) # 再取得する。reloadではpasswordが残る。
    assert_nil account.password
    assert account.valid?

    account.password = ""
    assert account.valid?
    assert !account.password_digest_changed?
    assert_nil account.password, "空文字列は設定できない。"

    account.password = nil
    assert account.invalid?
    assert account.password_digest_changed?
    assert_nil account.password, "nilは設定できる。保存はできない。"
    assert_equal ["Password can't be blank"], account.errors.full_messages

    account.password = "!!!"
    assert account.invalid?
    assert_equal ["Password is invalid", "Password is too short (minimum is 8 characters)"], account.errors.full_messages
    account.password = "12345678"
    assert account.valid?
    assert account.save!
  end
end