[{"_id":"aaaaa","groups":[{"name":"group0","props":{"unknown1":"value1","unknown2":"value2","unknown3":"value3"}},{"name":"group1","props":{"unknown4":"value4","unknown5":"value5","unknown6":"value6"}}]}]

db.collection.aggregate([{$project:{"props":"$groups.props"}},{$unwind:"$props"},{$project:{"result":{$objectToArray:"$props"}}},{$match:{$and:[{"result.k":"unknown2","result.v":"value1"}]}},{"$project":{result:{"$arrayToObject":"$result"}}}])