存储的每个对象由两个实体组成：数据和元数据。
Each object stored is composed of two entities: data and metadata.
描述符也可以提供与注释相同的有关实体的元数据信息，如主键字段，以及对象之间的任何映射。
The descriptor can also provide the same metadata information about the entities as annotations, such as primary key field, and any mappings between objects.
如果没有标识符，我们就没有办法把这个实体表现为元数据。
Without an identifier, there is no way to represent an entity as metadata.
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