[{_id:1,properties:[{"document":"doc1","sheet":"sheet1","property":"property1"},{"document":"doc1","sheet":"sheet2","property":"property2"},{"document":"doc1","sheet":"sheet2","property":"property3"},{"document":"doc2","sheet":"sheet1","property":"property4"},]}]

db.collection.aggregate([{"$match":{"_id":1}},{"$unwind":"$properties"},{"$group":{"_id":{"document":"$properties.document","sheet":"$properties.sheet"},"data":{"$addToSet":"$properties.property"}}},{"$project":{"_id":0,"document":"$_id.document","result":{"sheet":"$_id.sheet","data":"$data"}}}])