var orderSchema = new mongoose.Schema({
 uid: { type: String, required: true }, // 用户id
 amount: { type: Number, required: true },
 oType: { type: Number, required: true }, // 订单类型
 status: { type: Number, required: true }, // 订单的状态:1完成 2未完成 3失效
})

var userSchema = new mongoose.Schema({
 phone: String,
 status: String,
 createdAt: Date,
 updatedAt: Date
})

order.find().populate({path: 'uid', model: User, select: '_id real_name phone bankcard'}).exec(function(err, order) {
 // order: {
 // uid: {
 // phone: '15626202254',
 // status: "expand",
 // createdAt: Date,
 // updatedAt: Date
 // },
 // amount: 5000,
 // oType: 2, // 订单类型
 // status: 1, // 订单的状态:1完成 2未完成 3失效
 // }
});

{ "_id" : 1, "item" : "abc", "price" : 12, "quantity" : 2 }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1 }
{ "_id" : 3 }

{ "_id" : 1, "sku" : "abc", description: "product 1", "instock" : 120 }
{ "_id" : 2, "sku" : "def", description: "product 2", "instock" : 80 }
{ "_id" : 3, "sku" : "ijk", description: "product 3", "instock" : 60 }
{ "_id" : 4, "sku" : "jkl", description: "product 4", "instock" : 70 }
{ "_id" : 5, "sku": null, description: "Incomplete" }
{ "_id" : 6 }

db.orders.aggregate([
 {
 $lookup:
 {
  from: "inventory",
  localField: "item",
  foreignField: "sku",
  as: "inventory_docs"
 }
 }
])