inserting new field with $avg mongodb

Question

Every time user rates some post from 1 to 5, that number is written (as property value) with userId as property name.

"starRatingByUser" : {
    "iZxSjCduTjfCQbmf9" : 3,
    "LvBr6a427ofuvXFMp" : 4,
    "gfhfhfh98rtgfXFft" : 5
}

Is it possible to insert (update) new field ("starRatingAverage") with average of all ratings every time users add or updates rating?

Example:

"starRatingAverage": 4,
"starRatingByUser" : {
    "iZxSjCduTjfCQbmf9" : 3,
    "LvBr6a427ofuvXFMp" : 4,
    "gfhfhfh98rtgfXFft" : 5
}

I have this method:

Recipes.update(
  { _id: recipeId },
  { $set: { ["starRatingByUser." + this.userId]: star }}
)

[example image] : https://i.sstatic.net/CuWbR.png

Answer 1

There is an option to update using aggregation in mongodb. Update-documents-with-aggregation-pipeline

• The following update query sets to two args. First is to find the document, second is to do the aggregation

In aggregation

• convert the object into array using $objectToArray to calculate the total
• get the size of the array using $size and calculate the total using $reduce
• $project to remove unwanted fields

the script is

db.colelction.updateOne(
  {_id:1},
[
  {
    $addFields: {
      stu: {
        "$objectToArray": "$starRatingByUser"
      }
    }
  },
  {
    $addFields: {
      size: {
        $size: "$stu"
      },
      total: {
        $reduce: {
          input: "$stu",
          initialValue: 0,
          in: {
            $add: [
              "$$this.v",
              "$$value"
            ]
          }
        }
      }
    }
  },
  {
    "$project": {
      starRatingByAverage: {
        "$divide": [
          "$total",
          "$size"
        ]
      },
      starRatingByUser: 1
    }
  }
])

This query is checked and working fine