How to reference a model that's not in project using mongoose

I am building a microservice for Referral System for this existing app which has it's main backend already running. This Microservice will be an add-on to that. I built a Referral Model which is like this :

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import mongoose from "mongoose";

const RefSchema = new mongoose.Schema({
    referralID : {
        type : String,
        required : true
    },
    userID : {
        type : mongoose.Types.ObjectId,
        ref : 'Customer',
        required : true
    },
    userStatus : {
        type : String,
        required : true
    },
    linkedAccounts : {
        type : [mongoose.Types.ObjectId],
        ref : 'Customer',
        default : []
    }
},{
    timestamps : true
});

const RefModel = mongoose.model('referrals', RefSchema);
export default RefModel;

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Now, As you can see the ref is set to "Customer", which is exactly how it's modelled in the Main server. When I try to populate anything in the referral objects, I get an error from NestJS which says

Schema hasn't been registered for model "Customer".
Use mongoose.model(name, schema)

If you have any idea what's gone wrong please let me know, also I looked in the compass, the collection name is "customers", so even by trying that I got the same error.

Try to import the Customer schema before defining the Ref schema, this will initialize the required model.

Also, you should use mongoose.Schema.Types.ObjectId in Schema declarations:

import mongoose from "mongoose";
import Customer from "path/to/customer-schema";

const RefSchema = new mongoose.Schema({
    referralID : {
        type : String,
        required : true
    },
    userID : {
        type : mongoose.Schema.Types.ObjectId,
        ref : 'Customer',
        required : true
    },
    userStatus : {
        type : String,
        required : true
    },
    linkedAccounts : {
        type : [mongoose.Schema.Types.ObjectId],
        ref : 'Customer',
        default : []
    }
},{
    timestamps : true
});

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