mandatory attribute-identifier 命令式属性标识符
identifier attribute 标识符属性 ; [计] 识别符属性
attribute of identifier [计] 标识符属性
If you want to introduce a new attribute, you should pick an identifier key that is not already assigned to another attribute.
如果希望引入一个新属性，那么应当挑选一个还没有指定给其他属性的标识符键。
If you're from a more traditional database background, you can think of them as a resource identifier, an attribute name, and an attribute value.
如果您来自比较传统的数据库背景，可能会把它们看作资源标识符、属性名称和属性值。
A typical relation, for instance, would associate two classes with a matching identifier attribute.
比如，一个典型的关联会用一个匹配标志符属性联系两个类。
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