数据库中有OrderItems表，但是对象模型中并没有与之对应的持久化的类。
The object model has an Address class that has no corresponding table in the database, and the database has an OrderItems table that has no corresponding class.
定义一个支持持久化对象的对象模型的过程与定义任何对象模型相同。
Defining an object model to support persistent objects is the same process as defining any object model.
第二种是“以对象为中心”的方法，从业务模型和创建数据库来持久化这个模型开始。
The second is an "object-centric" approach, which starts with a business model and creates a database to persist the model.
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