How to group aggregate Fields in Layer 3

  mongodb, question

The following data format, I want tofill_timeFor group by,fill_timeFields are nested at the third level.
Business Requirements: If there are multiple different records in this recordfill_time, you need to group statistics many times. For example, the following data, statistics twice, respectively15 MarchAnd17 MarchOne for each. Can you use it, pleaseaggregateImplementation?

{
 "_id": "56e932f42ec56a4afc00a083",
 "binding_time": "2016-03-16 18:18:28",
 "categorys": [
 {
 "category_code": "003",
 "category_name": "cervical spondylosis",
 "datas": [
 {
 "fill_time": "2016-03-17 10:47:59",
 "form_code": "6",
 "plan_code": "P45",
 "results": []
 },
 {
 "fill_time": "2016-03-17 10:48:25",
 "form_code": "3",
 "plan_code": "P45",
 "results": []
 }
 ],
 "edc_patId": "P015830",
 "start_time": "2014-4-21 13:41:42"
 },
 {
 "category_code": "004",
 "category_name": "spinal tumor",
 "datas": [
 {
 "fill_time": "2016-03-15 10:48:48",
 "form_code": "34",
 "plan_code": "P47",
 "results": []
 },
 {
 "fill_time": "2016-03-15 10:49:14",
 "form_code": "36",
 "plan_code": "P47",
 "results": []
 }
 ],
 "edc_patId": "P038894",
 "start_time": "2014-4-21 13:41:42"
 }
 ],
 "patient_id": "000997785203",
 "person_name": "Zhang San",
 "project_code": "SYGK",
 "subscribe": false
 }
aggregate([
 {$unwind:"$categorys"},
 {$unwind:"$categorys.datas"},
 {$project:{
 'date':{$substr : ["$categorys.datas.fill_time",0,10]}
 }
 },
 {$group:{
 _id:'$date',
 "count":{$sum:1}
 }
 }])