Keys are the primary fields that tie relations together 键是将关系连接到一起的主字段。
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The methods on auto - increasment of primary fields of id in databse 中模型的数据表示方法
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A typical use is to specify the primary field of a data record so that at run time you can identify the data row displayed in the list 典型用法是指定数据记录的主字段,以便在运行时可以标识在列表中显示的数据行。