重写定义以减少嵌套级。
通过ActiveRecord，还可以嵌套：include选项，但嵌套深度只有一级。
With ActiveRecord, you can also nest the: include option, but only one level deep.
在这个嵌套、两级嵌入式方法示例中，您可以在自底向上方法中嵌入自顶向下方法(请参见图4)，或者反过来。
In the example of the nested, two-level embedding approach, you can embed the top-down approach within the bottom-up approach (see Figure 4) or the other way around.
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