How to use $ne in a $match along with $lookup in mongodb

Question

I have 3 collections as follows:

1. doctor.
2. professionalDetails
3. appointments

I would like to have all the available appointments for a doctor which are not in the appointments collection. I have tried the following query and been working on it from hours:

  const appointments = await Doctor.aggregate([
            {$match: {role: 'DOCTOR', _id: ObjectId(doctorId)}},
            {
              $lookup: {
                from: 'professionalDetails', 
                localField: '_id', 
                foreignField: 'doctorId', 
                as: 'professionalDetails'
              }
            },
            {
              $lookup: {
                from: 'appointments', 
                localField: '_id', 
                foreignField: 'doctorId', 
                as: 'appointments'
              }
            },
            {$unwind: "$professionalDetails"},
            {$unwind: "$professionalDetails.schedule"},
            {$match: {'professionalDetails.schedule.day': 'Tuesday'}},
            {$unwind: "$professionalDetails.schedule.timmings"},
            {$unwind: "$appointments"},
            {$match: {'professionalDetails.schedule.timmings._id': {$ne: 'appointments.slotId'}}},
            {
              $project: {
                _id: 0,
                time: '$professionalDetails.schedule.timmings.slot',
                _id: '$professionalDetails.schedule.timmings._id'
              }
            }
          ]);

The above still returns all the slots. However if I hardcode the not equal match as follows it works:

{$match: {'professionalDetails.schedule.timmings._id': {$ne: ObjectId('5dd2a9cab4a4a32c9359ac1f')}}}

Also both the ids are Object Ids in the mongoose model and not strings.

I am pretty new to it. Guidance is really appreciated!

Please help! Thanks in advance!

Answer 1

I tried adding $appointments.slotId as Tom suggested however that did not work out too.

After trying and looking around. Finally I got the output I was looking for after replacing the match statement with the following:

 {
      $match: { 
        $expr: {$ne: ['$professionalDetails.schedule.timmings._id', '$appointments.slotId']}
       }
  }

Any improvements or suggestions are still welcomed!