Mongodb update array of objects

Question

I have a document to Mongodb update array of objects like this:

{
  "_id" : 5,
  "quizzes" : [
     { "wk" : 1, "score" : 10 },
     { "wk" : 2, "score" : 8 },
     { "wk" : 5, "score" : 8 }
  ]
}

And I want to add new field in each object of quizzes array. The expected result should be

{
  "_id" : 5,
  "quizzes" : [
     { "wk" : 1, "score" : 10, "day": 1 },
     { "wk" : 2, "score" : 8, "day": 2 },
     { "wk" : 5, "score" : 8, "day": 3 }
  ]
}

Any solution for this.

how will you decide the order of the day?

Rahul Sharma
– Rahul Sharma

2018-03-15 06:29:32 +00:00
Commented Mar 15, 2018 at 6:29 — Rahul Sharma
– Rahul Sharma, Commented Mar 15, 2018 at 6:29

mickl · Accepted Answer · 2018-03-15 07:08:21Z

1

You can use Aggregation Framework:

db.col.aggregate([
    {
        $unwind: {
            path: "$quizzes",
            includeArrayIndex: "quizzes.day"
        }
    },
    {
        $group: {
            _id: "$_id",
            quizzes: { 
                $push: {
                    "score" : "$quizzes.score",
                    "wk" : "$quizzes.wk",
                    "day" : { $add: [ "$quizzes.day", 1 ] }
                }
            }
        }
    },
    {
        $out: "col"
    }
])

To assign indexes to each element you can use $unwind with includeArrayIndex option. Those indexes start with 0 so we have to use $add to start with 1. Then you can group back by your _id property and use $out to save aggregation results to your collection.

edited Mar 15, 2018 at 7:08

answered Mar 15, 2018 at 6:41

mickl

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2 Comments

Dinesh Sathrasala Over a year ago

can I do it without $project

mickl Over a year ago

@DineshSathrasala without $project you'll get 0,1,2 instead of 1,2,3 however you can move that logic into $group like in edited answer

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