title: Mongoose populate virtual field with a getter function is not working
body: <p>i am populating a virtual field in mongoose schema with a getter function.
  It should populate the field without any errors but it is throwing error</p>
  
  <p>MongooseError: If you are populating a virtual, you must set the localField
  and foreignField options</p>
  
  <p>when we are populating a field with getter then the localfield and foriegn field is not required</p>
  
  <pre><code>const DashboardSchema = new Schema({
      userId: {
          type: SchemaTypes.ObjectId
      }
  }, { toJSON: { virtuals: true } });
  
  DashboardSchema.virtual('TopReports').get(function () {
     return TopReports.find({ userId: this.userId }).sort('-date').limit(10);
  })
  </code></pre>