Ereditare un oggetto in Ruby

class Database
       attr_reader :db
       def initialize
               @db = SQLite3::Database.new('f.db')
       end
end
class Test < Database
       def initialize
               super
       end
       
       def do # doesn't work with @db and super.db
               @db.execute("sample_query") # super.db.execute("sample_query")
       end
end

class Database
       attr_reader :db
       def initialize
               @db = SQLite3::Database.new('social.db')
               init_db
       end
       
       def init_db
               @db.execute('CREATE TABLE IF NOT EXISTS user (id INTEGER PRIMARY KEY, name TEXT, surname TEXT, nickname TEXT, email TEXT, password TEXT, gender INTEGER)')
               @db.execute('CREATE TABLE IF NOT EXISTS post (id INTEGER PRIMARY KEY, sender TEXT, recipient TEXT, timestamp TEXT)')
       end
end
class User < Database
       attr_accessor :id, :name, :surname, :nickname, :email, :password, :gender
       def initialize(id, name, surname, nickname, email, password, gender)
               @id = id
               @name = name
               @surname = surname
               @nickname = nickname
               @email = email
               @password = password
               @gender = gender
       end
       
       def User.save(user)
               for user in u
                       @db.execute("INSERT INTO user(name, surname, nickname, email, password, gender) VALUES('#{u.name}', '#{u.surname}', '#{u.nickname}', '#{u.email}', '#{u.password}', '#{u.gender}')")
               end
       end
       
       def User.count
               @db.get_first_value("SELECT COUNT(*) FROM user")
       end
end
class Post < Database; end