Mongodb search field that is reference - mongodb

Question

New beginner of MongoDB here, I'm using sails.js(based on expressjs)/waterline(ORM)/MongoDB for this project. I have 2 collections, message and user. In any documents in message, there's this field called author that stores a reference/objectId of a user model:
module.exports = {
autoCreatedAt: true,
autoUpdatedAt: true,
schema: true,
tableName: 'message',
attributes: {
from: {
model: 'user',
required: true
},
to: {
model: 'user',
required: true
},
content: {
type: 'string',
maxLength: 288,
required: true
}
}
};
My user collection goes like this:
module.exports = {
autoCreatedAt: true,
autoUpdatedAt: true,
schema: true,
tableName: 'user',
attributes: {
email: {
type: 'string',
email: true,
unique: true,
required: true
},
name: {
type: 'string',
maxLength: 30
},
username: {
type: 'string',
unique: true,
required: true,
maxLength: 80
},
role: {
type: 'string',
required: true
},
......
},
};
Now I want to do a search query to all messages, given a search keyword, I want to show the paginated results of messages which the author's username and content of the message contains the keyword, how can I achieve that?
By the way, I know I can simply store username instead of user reference id in message documents, but says from Mongodb docs, objectId gives way better performance than string so...