Virtual Populating Embedded Docs with Dynamic "ref"

**Do you want to request a *feature* or report a *bug*?**
This is a request for clarification on usage or feature if it's currently not possible

**What is the current behavior?**
Unable / unclear on how to perform a virtual population of embedded docs with dynamic "ref"

**If the current behavior is a bug, please provide the steps to reproduce.**

```js
const mongoose = require('mongoose')
mongoose.connect('mongodb://localhost/mongoosetest')
mongoose.connection.once('open', () => {
const Schema = mongoose.Schema

// Generate Book Model
const bookSchema = new Schema({ productId: Number, name: String })
const BookModel = mongoose.model('Books', bookSchema, 'books')

// Generate Magazine Model
const magazineSchema = new Schema({ productId: Number, name: String })
const MagazineModel = mongoose.model('Magazines', magazineSchema, 'magazines')

// Purchases Schema
const purchasesSchema = new Schema({
refKey: String, // This will be either "Books" or "Magazines"
productId: Number
})

// User Schema
const userSchema = new Schema({
name: String,
purchases: [ purchasesSchema ]
},
{
toJSON: { virtuals: true },
toObject: { virtuals: true }
})

// Add virtual for population of embedded field with dynamic ref
userSchema.virtual('purchases_$', {
ref: doc => {
console.log('DOC', JSON.stringify(doc, null, 2))
/*
DOC "{
"_id": "5ad000068fc2370c7713c4bd",
"name": "John Doe",
"purchases": [
{
"_id": "5ad000068fc2370c7713c4bf",
"productId": 11,
"refKey": "Books"
},
{
"_id": "5ad000068fc2370c7713c4be",
"productId": 22,
"refKey": "Magazines"
}
],
"__v": 0
}"

The full document is provided here but how do I target
the "refKey" field of each embedded document in the purchases
array?
*/
return doc.purchases.refKey
},
localField: 'purchases.productId',
foreignField: 'productId'
})

// Create User Model
const UserModel = mongoose.model('Users', userSchema, 'users')

// Generate Items
const book = { productId: 11, name: 'Harry Potter' }
const magazine = { productId: 22, name: 'Time' }
const user = {
name: 'John Doe',
purchases: [
{ productId: 11, refKey: 'Books' },
{ productId: 22, refKey: 'Magazines' }
]
}

Promise.all([UserModel.create(user), MagazineModel.create(magazine), BookModel.create(book)])
.then(function () {
UserModel
.findOne({ name: 'John Doe' })
.populate('purchases_$')
.lean()
.then(results =>
console.log(JSON.stringify(results, null, 2))

/*
{
"_id": "5ad000068fc2370c7713c4bd",
"name": "John Doe",
"purchases": [
{
"_id": "5ad000068fc2370c7713c4bf",
"productId": 11,
"refKey": "Books"
},
{
"_id": "5ad000068fc2370c7713c4be",
"productId": 22,
"refKey": "Magazines"
}
],
"__v": 0
}
*/
)
})
})
```
**What is the expected behavior?**
I was expecting the virtual```purchases_$``` to be populated with "Harry Potter" and "Time" documents

```js
/*
{
"_id": "5ad000068fc2370c7713c4bd",
"name": "John Doe",
"purchases": [
{
"_id": "5ad000068fc2370c7713c4bf",
"productId": 11,
"refKey": "Books"
},
{
"_id": "5ad000068fc2370c7713c4be",
"productId": 22,
"refKey": "Magazines"
}
],
"purchases_$": [
{
"_id": "5ad000068fc2370c7713c4ba",
"productId": 11,
"name": "Harry Potter"
},
{
"_id": "5ad000068fc2370c7713c4bb",
"productId": 22,
"name": "Time"
}
],
"__v": 0
}
*/
```
Please let me know if this is the correct to populate or if there is a different way that you would recommend.

Thanks!

**Please mention your node.js, mongoose and MongoDB version.**
node: 9.7.1
mongoose: 5.0.14
mongo: 3.2.5

Start with the `purchases_$` virtual, its dynamic `ref`, and the `.populate('purchases_$')` call in the reproduction. Determine whether the embedded `refKey` values can select the Books and Magazines models independently, with the expected populated array as the completion criterion.

Written by the indexing model from the issue text.