Using mongoose middleware to add async virtuals - node.js

Question

In a node.js / Mongoose project, I have a schema which contains references to external image files.
var PageSchema = new Schema({
title: String
, media: {
digest: String
, name: String
}
});
Those files have additional properties which are stored in the file itself: url, width, height, exif fields, etc. Those fields will need to be populated before the model being sent to res.render().
For some fields, things are synchronous and a virtual just does the job:
PageSchema.virtual('media.url').get(function () {
return appPaths.fileUrl(this.media);
});
However, width / height, or exif fields require async calls. I thought of using middleware to populate them, but this does not seem to work:
PageSchema.post('init', function(next) {
var media = this.media;
var fileName = filedb.absoluteFilePath(media);
im.identify(fileName, function(err, features) {
if (err) {
media.width = 0;
media.height = 0;
} else {
media.width = features.width;
media.height = features.height;
}
next();
});
});
What am I doing wrong? Is there a common design pattern for solving this kind of problem? (Other than duplicating this information in the database itself?)

Answer 1

The real problem here is that mongoose currently seems to have a wonky implementation of post callbacks. While pre('init',function(next){ ... }); works as you expect, post('init',function(next){ ... }); does not actually get passed a next function. In fact, the post init callback does not receive any arguments whatsoever when it is called.
As such, I usually write a wrapper for my query callbacks to make a sort of DIY middleware:
var setAsyncVirtuals = function(callback){
return function(err, docs){
if(err) return callback(err);
var i = done = docs.length;
if(i > 0)
while(i--){
(function(i){
var filename = getFilename();
im.identify(filename, function(err, features) {
if (err) {
docs[i].media.width = 0;
docs[i].media.height = 0;
} else {
docs[i].media.width = features.width;
docs[i].media.height = features.height;
}
done--;
if(done <= 0) callback(null, docs);
});
})(i); // bind i to hold value for async call
}
else callback(null, docs);
}
}
then
Page.find({}, setAsyncVirtuals(function(err,docs){
res.send(docs); // these have media.width & media.height assigned
}));