[{"_id":"5f351f3d9d90b1281c44c5dp","staffId":12345,"category":"trend","page_route":"http://example.com/rer","expireAt":"2020-08-13T11:08:45.196Z","createdAt":"2020-08-13T11:08:45.199Z","updatedAt":"2020-08-13T11:08:45.199Z","__v":0},{"_id":"5f351f3d9d90b1281c44c5dö","staffId":12345,"category":"trend","page_route":"http://example.com/rer","expireAt":"2020-08-13T11:08:45.196Z","createdAt":"2020-08-13T11:08:45.199Z","updatedAt":"2020-08-13T11:08:45.199Z","__v":0},{"_id":"5f351f3d9d90b1281c44c5de","staffId":12346,"category":"incident","page_route":"http://example.com/rergfhfhf","expireAt":"2020-08-12T11:08:45.196Z","createdAt":"2020-08-12T11:08:45.199Z","updatedAt":"2020-08-12T11:08:45.199Z","__v":0},{"_id":"5f351f3d9d90b1281c44c5dc","staffId":12347,"category":"trend","page_route":"http://example.com/rerrwe","expireAt":"2020-08-13T11:08:45.196Z","createdAt":"2020-08-13T11:08:45.199Z","updatedAt":"2020-08-13T11:08:45.199Z","__v":0},{"_id":"5f351f3d9d90b1281c44c5dr","staffId":12348,"category":"trend","page_route":"http://example.com/rerrwe","expireAt":"2020-08-12T11:08:45.196Z","createdAt":"2020-08-12T11:08:45.199Z","updatedAt":"2020-08-12T11:08:45.199Z","__v":0}]

db.collection.aggregate([{$match:{"category":"trend","createdAt":{$regex:"2020-08-13"}}},{"$group":{"_id":{category:"$category"},"staffIds":{"$addToSet":"$staffId"}}}])