//多集合关联查询，product集合关联orders集合，使用$lookup做集合之间的关联
//集合之间设计不太合理，说明问题即可
/* product集合 */
{
"_id" : 1.0,
"name" : "草莓",
"type" : "水果"
}
{
"_id" : 2.0,
"name" : "苹果",
"type" : "水果"
}
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/* orders集合 */
{
"_id" : "1",
"pid" : 1.0,
"ordername" : "订单1"
}
{
"_id" : "2",
"pid" : 2.0,
"ordername" : "订单2"
}
{
"_id" : "3",
"pid" : 2.0,
"ordername" : "订单3"
}
{
"_id" : "4",
"pid" : 1.0,
"ordername" : "订单4"
}
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db.product.aggregate([{
$lookup:{
from: "oders",
localField: "_id", //指product的id字段
foreignField: "pid", //指product的id字段在orders集合中的外键字段
as: "results" //结果集名称
}
}])