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.

• Rahul Sharma: how will you decide the order of the day?

Answer 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.

• Dinesh Sathrasala: can I do it without $project
• mickl: @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