Is there a way to count distinct values in mongoose with specific cut off date

For example, I have the following two docs

[
   {
    "_id": "5fc534505144dd0030c44f8e",
    "createdAt": "2020-12-14T15:11:21.327Z"
    "user_id": "2",
  },
  {
    "_id": "5fc534505144dd0030c44f8e",
    "createdAt": "2020-12-14T14:10:40.427Z",
    "user_id": "1"
  },
  {
    "_id": "5fc534595144dd0030c44f95",
    "createdAt": "2020-12-13T14:10:58.027Z",
    "user_id": "1"
  }
]

the results should be

[
  {
    "date": "2020-12-13",
    "count":1
  },
  {
    "date": "2020-12-14",
    "count":2
  }
  ]

where the count is the number of distinct docs via user_ids till the date that specific cut off date

given data

data=[
   {
    "createdAt": "2020-12-14T15:11:21.327Z",
    "user_id": "2",
  },
  {
    "createdAt": "2020-12-14T14:10:40.427Z",
    "user_id": "1"
  },
  {
    "createdAt": "2020-12-13T14:10:58.027Z",
    "user_id": "1"
  },{
    "createdAt": new Date("2020-12-14T14:10:58.027Z"),
  }
]
> db.dummy.insert(data)

You may use aggregate: use $group with _id being the date's day in conjunction with $sum)

> db.dummy.aggregate({$group:{_id:{$substr:['$createdAt', 0, 10]}, count:{$sum:1}}})
{ "_id" : "2020-12-14", "count" : 3 }
{ "_id" : "2020-12-13", "count" : 1 }

edit: mongoose wise same may hold

const mongoose = require('mongoose')
mongoose.connect('mongodb://localhost:27017/dummy')

const UDate = mongoose.model('X', { createdAt:String, user_id: String }, 'dummy')

;(async()=>{
  mongoose.set('debug', true)
  const group = {$group:{_id:{$substr:['$createdAt', 0, 10]}, count:{$sum:1}}}
  const v = await UDate.aggregate([group])
  console.log('s : ', JSON.stringify(v))
  mongoose.disconnect()
})()

edit2: to handle unicity of userIds so there are not counted twice per date, you may use $addToSet instead of sum followed by a projection using $size

  const group = {$group:{_id:{$substr:['$createdAt', 0, 10]}, userIds:{$addToSet:'$user_id'}}}
  const count = {$project:{date:'$_id', count: {$size: '$userIds'}} }
  const v = await Stock.aggregate([group, count])

Lastly, if you feel always more, you can "rename" the _id field as date during the projection

{$project:{date:'$_id', _id:0, count: {$size: '$userIds'}} }