mongodb $lookup 联表查询
ollection orders : orders record example
collection items :items record example
$lookup 联表查询
通过item字段连接两个集合orders和items,然后使用 $replaceRoot 中的 $mergeObjects 合并成items和orders的连接文档
db.orders.aggregate( [{$lookup: {from: \"items\",localField: \"item\", // field in the orders collectionforeignField: \"item\", // field in the items collectionas: \"fromItems\"}},{$replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ \"$fromItems\", 0 ]