通常来说，模型类将包含帮助你对数据库进行增删改查的方法。
Typically your model classes will contain functions that help you retrieve, insert, and update information in your database.
PersistenceManager类包含了实现了基本的增删改查操作的方法。
The PersistenceManager class contains methods to implement all basic CRUD (create retrieve, update, and delete) operations.
服务表达相关的标准的一个例子是CRUD(增删改查)，传统上，它用于给组件配备一组可预见的方法。
An example of a standard associated with contract expression is the CRUD (create, read, update, delete) convention traditionally used to outfit components with a predictable set of methods.
服务表达相关的标准的一个例子是CRUD(增删改查)，传统上，它用于给组件配备一组可预见的方法。
An example of a standard associated with contract expression is the CRUD (create, read, update, delete) convention traditionally used to outfit components with a predictable set of methods.
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