能唯一标识一个元组的属性称乀为关键属性(Primary Key Attribute)戒简称为主关键字(Primary Key)。主关键字有时是由夗个属性组成的,此时的主关键字叨做组合关键字(Conˉcatenated primary Key)。
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Instead, it requires the developer only to denote the attributes to be persisted and to identify the attribute to represent the object's unique identifier (or primary key).
相反，它只需要程序员将属性标记为持久性的，并识别出能代表对象唯一标识符(或者主键)的属性。
They use database-generated primary key values for the ID attribute.
它们为 ID 属性使用数据库生成的主键值。
We will include this Key as an attribute of AcmeMembership, mark it as a Primary Key, and specify the strategy the Google App Engine runtime will use to populate it automatically with unique values.
我们将会将该Key作为AcmeMembership的属性添加，并将其标记为一个主键，并指定Google App Engine运行时的策略，它将会自动填充上唯一的值。
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