Mongoose Virtuals in MongoDB Aggregate

Question

My Mongoose Schema is as follows:

var DSchema = new mongoose.Schema({
  original_y: {type: Number},,
  new_y: {type: Number},,
  date: {type: Date},
  dummy         


        

Answer 1

A couple notes in the docs touch on why this is so:

• Arguments are not cast to the model's schema because $project operators allow redefining the "shape" of the documents at any stage of the pipeline, which may leave documents in an incompatible format.
• The documents returned are plain javascript objects, not mongoose documents (since any shape of document can be returned).

But it goes beyond this because the aggregate operation is performed server-side, where any client-side Mongoose concepts like virtuals do not exist.

The result is that you'll need to include the date field in your $project and $group stages and add your own dateformatted field to the results in code based on the date values.

Answer 2

the <field>: <1 or true> form is used to include an existing field which is not the case here since the dateformatted field doesn't exist and you have to create it using an expression, $dateToString can be used:

"$project": {
  "original_y": 1, 
  "new_y": 1,
  "dateformatted": { "$dateToString": { "format": "%Y-%m-%d %H:%M:%S", "date": "$date" } },
  ...

Another option is to use it with $addFields:

{
  "$project": {
    ...
  }
},
{
  "$addFields": {
    "dateformatted": { "$dateToString": {"format": "%Y-%m-%d %H:%M:%S", "date": "$date"} }
  }
},
...

Answer 3

This is an old question but I've come up with a useful hack to get back the virtuals and thought it might be useful for those searching for this problem.

You can easily convert the objects back to mongoose models:

documents = documents.map(d => {
  return new Document(d);
});

var virtual = documents[0].virtualProperty;