默认情况下，该类为空，因为Rails将根据来自数据库的表元数据推断该类的属性。
The class is empty by default, as Rails infers the attributes of the class based on table metadata from the database.
根据价格表，他们花了45元买门票。
According to the price list, they paid 45 yuan for the tickets.
有些人认为词汇表和分类法也属于元数据的范畴。
Some people consider glossary and taxonomy as metadata as well.
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