const UsersSchema = Schema({
    name: String,
    email: {type: String, unique: true, lowercase: true},
    password: {type: String, select: false},
    created: {type: Date, default: Date.now()},
});
module.exports = mongoose.model('User', UsersSchema);

const ReferralsSchema = Schema({
    image: String,
    userId: {type: String, select: false},
    created: {type: Date, default: Date.now()}
});
module.exports = mongoose.model('Image', ImageSchema);

ImageSchema.aggregate()
   .lookup(
            { from: 'users', localField: 'userId', foreignField: '_id', as: 'user' }
   )
   .exec( (err, images) => {
            console.log(err,images);
})

{ _id: 5b0ed0a671dd9a3c7ac115f5,
    image: 'testigimage.png',
    userId: '5aa97c526fc4e7108738e8c1',
    __v: 0,
    user: [] } ]