ActiveRecord会为不同的任务变量使用主键和副键，它们都是作为数据库的约束而定义的。
ActiveRecord USES primary and foreign keys for a variety of tasks, and both are defined as constraints on your database.
座椅中间宽阔的副仪表台底下安装了电池组，这是整车的心脏部位，含有各种开关和控制键。
The large central tunnel which houses the batteries is undoubtedly the heart of the vehicle and contains all the switches and controls.
安装时先安装预应力拉杆副，然后施加预应力，最后安装抗剪键。
When in installation, the prestressed pull rod pairs are firstly installed, then the prestress is applied, and the shear keys are finally installed.
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