Populate() specify multiple model - node.js

Question

I'm looking to make a populate() on a find() request but specifying multiple models, so if no occurrence is found on the first specified model the population will make on the second.
Here is an example of what I would like to do :
const userSch = new Schema({
username: {
type: String,
required: true,
},
password: {
type: String,
required: true,
},
mail: {
type: String,
required: true,
unique: true,
}
});
const botSch = new Schema({
username: {
type: String,
required: true,
},
token: {
type: String,
required: true,
unique: true,
},
owner: mongoose.Schema.Types.ObjectId
});
const nspSch = new Schema({
name: String,
channels: [channels],
users: [{
id: {
type: mongoose.Schema.Types.ObjectId,
},
bot: {
type: Boolean,
}
}],
});
const channels = new Schema({
name: String,
message: [{
user: {
type: Schema.Types.ObjectId,
},
content: String,
}],
});
const nsp = mongoose.model('namespace', nspSch);
const Auth = mongoose.model('users', userSch);
const BotAuth = mongoose.model('bots', botSch);
function returnMs(nspID, channelID, callback) {
nsp.findOne({'_id': nspID}).populate({
path: 'channels.message.user',
model: 'users' || 'bots',
select: '_id username',
})
.exec(function(err, r) {
...
})
}
If there is ever an npm package, or a solution or even a track to code it, please share it.
Thank you