I am trying to get result from mongoose populate but it giving null result?

Working with Data Node.js Frameworks
, ,
1 
app.post("/showmaterial",(req,res)=>{
    purchaseHeaderModel.findOne({
    "vendor_code_id.vendor_code":"1"
    }).populate("vendor_code_id")
    .then((found)=>{
        res.json(found)
    })
    .catch((err)=>res.json(err))
}) //calling api 

// these are the schema and showing null result and i am unable to find the solution

const mongoose=require("mongoose");
const vendorSchema=new mongoose.Schema({
    vendor_code:{
        type:String,required:true,unique:true
    },
    vendor_name:{
        type:String,
        required:true
    }
});

const purchaseHeaderSchema=new mongoose.Schema({
    po_no:{
        type:String,required:true,unique:true
    },
    vendor_code_id:{
        type:mongoose.Schema.Types.String,
        ref:"vendor_data",
        required:true
    }
})[quote="Parmesh_Kumar, post:1, topic:208209, full:true"]
app.post("/showmaterial",(req,res)=>{
    purchaseHeaderModel.findOne({
    "vendor_code_id.vendor_code":"1"
    }).populate("vendor_code_id")
    .then((found)=>{
        res.json(found)
    })
    .catch((err)=>res.json(err))
}) //calling api 

// these are the schema and showing null result and i am unable to find the solution

const mongoose=require("mongoose");
const vendorSchema=new mongoose.Schema({
    vendor_code:{
        type:String,required:true,unique:true
    },
    vendor_name:{
        type:String,
        required:true
    }
});

const purchaseHeaderSchema=new mongoose.Schema({
    po_no:{
        type:String,required:true,unique:true
    },
    vendor_code_id:{
        type:mongoose.Schema.Types.String,
        ref:"vendor_data",
        required:true
    }
})