兩張表格answer和question.

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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，字段信息來自于另外一張表（這篇博客屬于這種情況）

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查詢命令如下:

db.getCollection('question').aggregate([
{'$match': {'_id': {'$in': [ObjectId("5e9ed4d8812118ab9078ae1b"),ObjectId("5e9ed5761bd54499b6cce47b")]}}},
{'$lookup':{
? ? 'from':'answer',
? ? 'localField':'_id',
? ? 'foreignField':'question_id',
? ? 'as':'answer_list'}}])

上述命令的意思是:

新增一個字段，名字叫answer_list

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{'_id': {'$in': [xxx,yyy]}}}就是下面的? ? 'localField':'_id',

列表[]中填寫的是表格question中的_id的取值

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? ? 'from':'answer'的意思是:

從answer這張表格中去尋找信息來補充當前的question表的查詢結果．

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表格名稱	映射的字段
question	_id
answer	question_id

?

總結

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