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 ]