$lookup做关联表查询

两张表格answer和question.

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关联表的查询，一般无非两种情况:

两张表合二为一，

其中一张表增加一些字段，字段信息来自于另外一张表（这篇博客属于这种情况）

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查询命令如下:

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:

新增一个字段，名字叫answer_list

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

列表[]中填写的是表格question中的_id的取值

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    'from':'answer'的意思是:

从answer这张表格中去寻找信息来补充当前的question表的查询结果．

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表格名称	映射的字段
question	_id
answer	question_id

 

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