[{"_id":ObjectId("5d12b2a10507cfe0bad6d93c"),"participants":[{"_id":ObjectId("5ce4af580507cfe0ba1c6f5b"),"firstname":"John","lastname":"Anderson","icon":"/assets/images/avatars/small/2.jpg"},{"_id":ObjectId("5ce4af580507cfe0ba1c6f5b"),"firstname":"John","lastname":"Anderson","icon":"/assets/images/avatars/small/2.jpg"}]},{"_id":ObjectId("5d1124a50507cfe0baba7909"),"participants":[{"_id":ObjectId("5ce4af580507cfe0ba1c6f5b"),"firstname":"John","lastname":"Anderson","icon":"/assets/images/avatars/small/2.jpg"},{"_id":ObjectId("5ce54cb80507cfe0ba25d74b"),"firstname":"Milosh","lastname":"Jersi","icon":"/assets/images/avatars/small/3.jpg"}]}]

db.collection.aggregate([{"$unwind":"$participants"},{"$group":{"_id":"$_id","participants":{"$addToSet":"$participants._id"}}},{"$match":{"participants":{"$size":1}}}])