Conditional update, depending on field matched - node.js

Question

Say I have a collection of documents, each one managing a discussion between a teacher and a student:
{
_id,
teacherId,
studentId,
teacherLastMessage,
studentLastMessage
}
I will get queries with 3 parameters: an _id, a userId and a message.
I'm looking for a way to update the teacherLastMessage field or studentLastMessage field depending on which one the user is.
At the moment, I have this:
return Promise.all([
// if user is teacher, set teacherLastMessage
db.collection('discussions').findOneAndUpdate({
teacherId: userId,
_id
}, {
$set: {
teacherLastMessage: message
}
}, {
returnOriginal: false
}),
// if user is student, set studentLastMessage
db.collection('discussions').findOneAndUpdate({
studentId: userId,
_id
}, {
$set: {
studentLastMessage: message
}
}, {
returnOriginal: false
})
]).then((results) => {
results = results.filter((result) => result.value);
if (!results.length) {
throw new Error('No matching document');
}
return results[0].value;
});
Is there a way to tell mongo to make a conditional update, based on the field matched? Something like this:
db.collection('discussions').findOneAndUpdate({
$or: [{
teacherId: userId
}, {
studentId: userId
}],
_id
}, {
$set: {
// if field matched was studentId, set studentLastMessage
// if field matched was teacherId, set teacherLastMessage
}
});
Surely it must be possible with mongo 3.2?

Answer 1

What you want would require referencing other fields inside of $set. This is currently impossible. Refer to this ticket as an example.
First of all, your current approach with two update queries looks just fine to me. You can continue using that, just make sure that you have the right indexes in place. Namely, to get the best performance for these updates, you should have two compound indexes:
{ _id: 1, teacherId: 1 }
{ _id: 1, studentId: 1 }.
To look at this from another perspective, you should probably restructure your data. For example:
{
_id: '...',
users: [
{
userId: '...',
userType: 'student',
lastMessage: 'lorem ipsum'
},
{
userId: '...',
userType: 'teacher',
lastMessage: 'dolor sit amet'
}
]
}
This would allow you to perform your update with a single query.

Answer 2

Your data structure is a bit weird, unless you have a specific business case which requires the data the be molded that way i would suggest creating a usertype unless a user can both be a teacher and a student then keep your structure.
The $set{} param can take a object, my suggestion is to do your business logic prior. You should already know prior to your update if the update is going to be for a teacher or student - some sort of variable should be set / authentication level to distinguish teachers from students. Perhaps on a successful login in the callback you could set a cookie/local storage. Regardless - if you have the current type of user, then you could build your object earlier, so make an object literal with the properties you need based on the user type.
So
if(student)
{
var updateObj = { studentLastMsg: msg }
}
else
{
var updateObj = { teacherLastMsg: msg }
}
Then pass in your update for the $set{updateObj} I'll make this a snippet - on mobile