How to work with async code in Mongoose virtual pr

Question

I'm trying to work with associating documents in different collections (not embedded documents) and while there is an issue for that in Mongooose, I'm trying to work around it now by lazy loading the associated document with a virtual property as documented on the Mongoose website.

The problem is that the getter for a virtual takes a function as an argument and uses the return value for the virtual property. This is great when the virtual doesn't require any async calls to calculate it's value, but doesn't work when I need to make an async call to load the other document. Here's the sample code I'm working with:

TransactionSchema.virtual('notebook')
  .get( function() { // <-- the return value of this function is used as the property value
    Notebook.findById(this.notebookId, function(err, notebook) {
      return notebook; // I can't use this value, since the outer function returns before we get to this code
    })
    // undefined is returned here as the properties value
  });

This doesn't work since the function returns before the async call is finished. Is there a way I could use a flow control library to make this work, or could I modify the first function so that I pass the findById call to the getter instead of an anonymous function?

Answer 1

While this addresses the broader problem rather than the specific question, I still thought it was worth submitting:

You can easily load an associated document from another collection (having a nearly identical result as defining a virtual) by using Mongoose's query populate function. Using the above example, this requires specifying the ref of the ObjectID in the Transaction schema (to point to the Notebook collection), then calling populate(NotebookId) while constructing the query. The linked Mongoose documentation addresses this pretty thoroughly.

I'm not familiar with Mongoose's history, but I'm guessing populate did not exist when these earlier answers were submitted.

Answer 2

As this is an old question, I figured it might use an update.

To achieve asynchronous virtual fields, you can use mongoose-fill, as stated in mongoose's github issue: https://github.com/Automattic/mongoose/issues/1894

Answer 3

Josh's approach works great for single document look-ups, but my situation was a little more complex. I needed to do a look-up on a nested property for an entire array of objects. For example, my model looked more like this:

var TransactionSchema = new Schema({
  ...
  , notebooks: {type: [Notebook]}
});

var NotebookSchema = new Schema({
  ...
  , authorName: String  // this should not necessarily persist to db because it may get stale
  , authorId: String
});

var AuthorSchema = new Schema({
  firstName: String
  , lastName: String
});

Then, in my application code (I'm using Express), when I get a Transaction, I want all of the notebooks with author last name's:

...
TransactionSchema.findById(someTransactionId, function(err, trans) {
  ...
  if (trans) {
    var authorIds = trans.notebooks.map(function(tx) {
        return notebook.authorId;
      });

    Author.find({_id: {$in: authorIds}, [], function(err2, authors) {
      for (var a in authors) {
        for (var n in trans.notebooks {
          if (authors[a].id == trans.notebooks[n].authorId) {
            trans.notebooks[n].authorLastName = authors[a].lastName;
            break;
          }
        }
      }
      ...
    });

This seems wildly inefficient and hacky, but I could not figure out another way to accomplish this. Lastly, I am new to node.js, mongoose, and stackoverflow so forgive me if this is not the most appropriate place to extend this discussion. It's just that Josh's solution was the most helpful in my eventual "solution."

Answer 4

You can define a virtual method, for which you can define a callback.

Using your example:

TransactionSchema.method('getNotebook', function(cb) {
  Notebook.findById(this.notebookId, function(err, notebook) {
    cb(notebook);
  })
});

And while the sole commenter appears to be one of those pedantic types, you also should not be afraid of embedding documents. Its one of mongos strong points from what I understand.

One uses the above code like so:

instance.getNotebook(function(nootebook){
    // hey man, I have my notebook and stuff
});