兩張**answer和question.
關聯表的查詢，一般無非兩種情況:
兩張表合二為一，
其中一張表增加一些字段，字段資訊來自於另外一張表（這篇部落格屬於這種情況）
查詢命令如下:
db.getcollection('question').aggregate([
}},}])
上述命令的意思是:
新增乙個字段，名字叫answer_list
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}}就是下面的 'localfield':'_id',
列表中填寫的是**question中的_id的取值
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'from':'answer'的意思是:
從answer這張**中去尋找資訊來補充當前的question表的查詢結果．
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**名稱對映的字段
question
_idanswer
question_id
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