{
  username: "Lorem Ipsum",
  password: "Dolar Sit",
  company: "5c73afcf9a3bde1a40da5184"
}

{
  username: "Lorem Ipsum",
  password: "Dolar Sit",
  company: {
     company_name: "Blah Blah",
     company_address: "Blah Blah Blah",
     company_website: "BlahBlah.com",
  }
}

const Company = require('./company');
Schema = mongoose.Schema

// User Schema
const userSchema = mongoose.Schema({
    username:{
        type: String,
        required: true
    },
    password:{
        type: String,
        required: true
    },
    company:{
        type: [{ type: Schema.Types.ObjectId, ref: 'Company' }],
        required: true
    },
    created_on:{
        type: Date,
        default: Date.now
    },
    updated_on:{
        type: Date,
        default: Date.now
    }
});

const User = module.exports = mongoose.model('Users', userSchema);
// Add User
module.exports.addUser = (user, callback) => {
    User.create(user, callback);
}

const mongoose = require('mongoose');

// Book Schema
const companySchema = mongoose.Schema({
    company_name:{
        type: String
    },
    target_address:{
        type: String
    },
    target_website:{
        type: String
    },
    created_on:{
        type: Date,
        default: Date.now
    },
    updated_on:{
        type: Date,
        default: Date.now
    }
});

const Company = module.exports = mongoose.model('Company', companySchema);

// Add Company
module.exports.addCompany = (comp, callback) => {
    Company.create(comp, callback);
}

'Cast to Array failed for value "{ company_name: 'Moshi Moshi', company_address: 'Bengaluru' }" at path "company"',
        name: 'CastError',
        stringValue:
         '"{ company_name: 'Moshi Moshi', company_address: 'Bengaluru' }"',
        kind: 'Array',
        value: [Object],
        path: 'company',
        reason: [MongooseError] } },
  _message: 'Users validation failed',
  name: 'ValidationError' }
[nodemon] restarting due to changes...
[nodemon] starting `node app.js`
E:MoshiMoshiProjectsMaaamamiablackhole-restmodelsusers.js:17
        type: [{ type: ObjectId, ref: 'Company' }],
                       ^

ReferenceError: ObjectId is not defined

//incorrect
 company:{
    type: [{ type: Schema.Types.ObjectId, ref: 'Company' }],
    required: true
 },

 //correct
  company:{
     type: Schema.Types.ObjectId,
     ref: 'Company',
     required: true
 },

let companyObject=//your company object
  let userObject=//your user object

   let savedCompanyObject= await addCompanyObject.save() //save CompanyObject
   userObject.company = savedCompanyObject._id;

  await userObject.save() //now save user object