An object that wraps a record data structure in an external resource, such as a row in a database table, and adds some domain logic to that object.
Active Record uses the most obvious approach: put data access logic into the domain object. This way all people know how to read and write their data from the database.
把各自的CRUD操作都写在Domain Object的类里面，这样做的好处是减少 Transaction Script的体积，因为 各个演员自己会做持久化，不用麻烦总导演了