python - How can i choose unique users in mongodb -

i have database events have happened in game. trying retrieve information how many unique players have started level. use piece of code that:

heh = list(db.events.aggregate( [     { "$match": {"status": 'start'}},     {"$group": {"_id": "$eventname", "players": {"$sum": 1}}}, ])) print(heh) 

but getting information how many times level started. how can change code right info? unique users have unique "uid".

try this.. $addtoset adds unique records , count records in array

db.events.aggregate( [    { "$match": {"status": 'start'}},    {"$group": {"_id": "$eventname", "players": {"$addtoset": "$uid"}}},    {"$project": {"_id": 1, "count": {"$size": "$players"}}} ]) 

db.test1.aggregate(  [          { "$match": {"status": 'start'}},          {"$group": {"_id": "$eventname", "players": {"$addtoset": "$uid"}}},     {$unwind:"$players"},     {$group:{_id:"$_id",count:{$sum:1}}} ] )