$ rails _8.0.1_ new hoge_app
$ cd hoge_app
$ rails g model vehicle type:string color:string price:integer release_date:datetime
$ rails g model car --parent=Vehicle
$ rails db:migrate

# Gemfile
source "https://rubygems.org"

gem "rails", "~> 8.0.1"
gem "default_value_for"

# app/models/vehicle.rb
class Vehicle < ApplicationRecord
  default_value_for :color, "black"
end

# app/models/car.rb
class Car < Vehicle
  default_value_for :price, 100
  default_value_for :release_date, DateTime.now
end

$ bundle install
$ rails r 'p Car.new'
#<Car id: nil, type: "Car", color: "black", price: nil, release_date: "2024-12-26 04:30:36.002660000 +0000", created_at: nil, updated_at: nil>

$ rails r 'p Car.new'
#<Car id: nil, type: "Car", color: "black", price: 100, release_date: "2024-12-26 04:30:36.002660000 +0000", created_at: nil, updated_at: nil>