Mongoose ('findOneAndUpdate') Middleware: Need access to original document - mongodb

Question

I am attempting to use pre('findOneAndUpdate') to update the icon attribute of the Meeting document. The update is based on the pre-existing value of the yearlymeeting attribute (see below).
Because pre and post save() hooks are not executed on update(), I seem to be unable to access the original document at all. Yet this is critical for the operation I'm trying to perform. Is there any way around this?
For example, I am able to accomplish my purpose on pre('save'), like so:
meetingSchema.pre('save', function(next) {
const yearlymeetingSlug = this.yearlymeeting[0].toLowerCase().replace(/[^A-z0-9]/g, '');
this.icon = `${yearlymeetingSlug}.png`
next();
});
What I would like to be able to do is something like this:
meetingSchema.pre('findOneAndUpdate', function(next) {
const yearlymeetingSlug = originalDocument.yearlymeeting[0].toLowerCase().replace(/[^A-z0-9]/g, '');
this.icon = `${yearlymeetingSlug}.png`
next();
});
I understand that this in pre(findOneAndUpdate) refers to the query, rather than the stored document itself. Is there any way to access the document, so that I can update icon based on the stored value of yearlymeeting?

Answer 1

tl;dr
Not possible via middleware. Query for the doc first, and then separately update a specific version of the doc to prevent race conditions.
Can't do it the way you're trying according to this issue on the Mongoose Github (from the main dev):
By design - the document being updated might not even be in the server's memory. In order to do that, mongoose would have to do a findOne() to load the document before doing the update(), which is not acceptable.
The design is to enable you to manipulate the query object by adding or removing filters, update params, options, etc. For instance, automatically calling .populate() with find() and findOne(), setting the multi: true option by default on certain models, access control, and other possibilities.
findOneAndUpdate() is a bit of a misnomer, it uses the underlying mongodb findAndModify command, it's not the same as findOne() + update(). As a separate operation, it should have its own middleware.
Following this, there are no other suggestions in the issue thread to access the original document inside of the middleware itself.
What I've seen done (and what I've had to do many times myself), is simply have to query for the document before updating it (which, of course, could lead to a race condition depending on who is updating the doc, and when, but you can fix that by also querying for a specific version of the document -- a sort of "optimistic locking"):
let meeting = yield Meeting.findOne({}).exec()
let update = {}
// ... some conditional logic to figure out which icon to set
update.icon = // whatever
yield Meeting.update({ _id: meeting._id, version: meeting.version }, update)
This is of course assuming you have a "version" field in your schema. This sort of locking will prevent you from updating an old version of the doc. If you're gonna use this kind of versioning, you'll also probably want to add some middleware that updates the version of a doc any time the doc is updated/saved.
You can also use a more naïve implementation, where you don't use locking, which may be fine in your specific business case, as long as you're aware of the possibility of a race condition, and the risks.

Answer 2

This may not be the best solution, but I did find a way to make it work. I used the controller rather than schema pre hooks. Here's what my update controller looks like now:
exports.updateMeeting = async (req, res) => {
const _id = req.params.id
let meeting = await Meeting.findOneAndUpdate({ _id }, req.body, {
new: true,
runValidators: true
});
/* New Code: */
const yearlymeetingSlug = meeting.yearlymeeting[0].toLowerCase().replace(/[^A-z0-9]/g, '');
meeting.icon = `${yearlymeetingSlug}.png`;
meeting.save();
req.flash('success', 'meeting successfully updated!');
res.redirect(`/meetings/${meeting.slug}`);
};
I welcome your feedback on any problems you see with this solution.