一对多
先创建一张表countryCode
db.countryCode.insert([{code: 1}, {code: 20}, {code: 30}])
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第二张表countryCodeLookup，这张表显示country的code和country的name 的对应关系是一对多
db.countryCodeLookup.insert([{code: 1, name: "United States"}, {code: 20, name: "Egypt"},
{code: 1, name: "Foobar"}, {code: 30, name: "Greece"}])
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$lookup、$project
下面通过 $lookup 操作符连接两个集合，其中 $project 会过滤属性
db.countryCode.aggregate([
{ $lookup: {from: "countryCodeLookup", localField: "code", foreignField: "code", as: "countryName"} },
{ $project: {"code":1, "countryName.name":1, "_id":0} }
])
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$match
$match 会查询连接后的表，显示符合条件的。
db.countryCode.aggregate([
{ $lookup: {from: "countryCodeLookup", localField: "code", foreignField: "code", as: "countryName"} },
{ $project: {"code":1, "countryName.name":1, "_id":0} },
{ $match:{"code":1.0}}
])
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一对一
$unwind
$unwind操作符会分解$lookup中的as数组，使之扁平化返回。
db.countryCode.aggregate([
{ $lookup: {from: "countryCodeLookup", localField: "code", foreignField: "code", as: "countryName"} },
{ $project: {"code":1, "countryName.name":1, "_id":0} },
{ $unwind: "$countryName"},
{ $match:{"code":1.0}}
])
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现在countryName是一个子文档，在project中设置把子文档中的属性拿出来，这里$project要在$unwind属性后面。
db.countryCode.aggregate([
{ $lookup: {from: "countryCodeLookup", localField: "code", foreignField: "code", as: "countryName"} },
{ $unwind: "$countryName"},
{ $project: {"code":1, "name": "$countryName.name", "_id":0} }
])
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