[{teacher:{userId:1},students:[{"_id":"5e84d1a1ab3ebf54283b8cb2","userId":"5dd27452592f600900235945","name":"student zero","profile_picture":"https://productionstemulistorage.blob.core.windows.net/stemuli/profile-picture-6e609f3b-44cb-44c0-888a-1b6767e3072d"},{"_id":"5e84d1a1ab3ebf54283b8cb4","userId":"5dd27452592f600900235945","name":"student zero","profile_picture":"https://productionstemulistorage.blob.core.windows.net/stemuli/profile-picture-6e609f3b-44cb-44c0-888a-1b6767e3072d"}]}]

db.collection.aggregate([{$match:{"teacher.userId":1}},{$project:{"students._id":0}},{$unwind:"$students"},{$group:{_id:null,students:{$addToSet:"$students"}}}])