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 collection
         foreignField: "item",  // field in the items collection
         as: "fromItems"
      }
   },
   {
      $replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$fromItems", 0 ]