[{"key":1},{"key":2}]

db.collection.aggregate([{$lookup:{from:"users",localField:"user",foreignField:"_id",as:"user",let:{consultation:"$medicalPractice.consultation"},pipeline:[{"$addFields":{"slots":{$reduce:{input:"$$consultation",initialValue:[],in:{$concatArrays:["$$value",{$map:{input:"$$this.slots",as:"slot",in:{slot:"$$slot"}}}]}}}}},{"$addFields":{slotsLength:{$size:"$slots"}}}]},},{$limit:10}])