how to use populate and aggregate in same statement?

With the latest version of mongoose (mongoose >= 3.6), you can but it requires a second query, and using populate differently. After your aggregation, do this:

Patients.populate(result, {path: "patient"}, callback);

See more at the Mongoose API and the Mongoose docs.

Edit: Looks like there's a new way to do it in the latest Mongoose API (see the above answer here: https://stackoverflow.com/a/23142503/293492)

Old answer below

You can use $lookup which is similar to populate.

In an unrelated example, I use $match to query for records and $lookup to populate a foreign model as a sub-property of these records:

  Invite.aggregate(
      { $match: {interview: req.params.interview}},
      { $lookup: {from: 'users', localField: 'email', foreignField: 'email', as: 'user'} }
    ).exec( function (err, invites) {
      if (err) {
        next(err);
      }

      res.json(invites);
    }
  );

You can do it in one query like this:

Appointments.aggregate([{
        $group: {
            _id: '$date',
            patients: {
                $push: '$patient'
            }
        }
    },
    {
        $project: {
            date: '$_id',
            patients: 1,
            _id: 0
        }
    },
    {
        $lookup: {
            from: "patients",
            localField: "patient",
            foreignField: "_id",
            as: "patient_doc"
        }
    }
])

populate basically uses $lookup under the hood. in this case no need for a second query. for more details check MongoDB aggregation lookup

You have to do it in two, not in one statement.

In async await scenario, make sure await until populate.

const appointments = await Appointments.aggregate([...]);

await Patients.populate(appointments, {path: "patient"});

return appointments;

or (if you want to limit)

await Patients.populate(appointments, {path: "patient",  select:  {_id: 1, fullname: 1}});