//bills collections
const mongoose = require('mongoose');  
const Schema = mongoose.Schema;  
// Define collection and schema for Product  
let bills = new Schema({  
  Id: { type: String },Name: { type: String },money: { type: Number },Perc: { type: String },Month: { type: String },branchName:{type: mongoose.Schema.Types.ObjectId,ref: 'branches'},},{  
    collection: 'bills'  
});  
module.exports = mongoose.model('bills',bills); 

//branches collection
const mongoose = require('mongoose');  
const Schema = mongoose.Schema;  
// Define collection and schema for Product  
let branches = new Schema({  
  branchName: {  
    type: String  
  },{  
    collection: 'branches'  
});  
module.exports = mongoose.model('branches',branches); 

如果我将Secunderabad作为branchName的branchName发送给账单集，则与分支集合_id中的Secunderabad相比，它保存了另一个ID 双子城是分支机构之一 然后我要成为相同的ID。