Update all a field in all document based on another field in that document

Question

I have student documents in my database. Each document is a student and has the fields amountOwed and tuition.

I am trying to update the amountOwed to be equal to the current value of amountOwed + tuition.

I've thought about using a .find query to get all the student documents, looping through each document and updating the database.

I've also researched and found the $set(aggregation) and $addFields(aggregation) in the mongo documentation but I'm not sure if I can use that for what I'm trying to do.

Example documents:

{"studentName" : "Student1",

 "amountOwed" : 100,

 "tuition" : 100

},

{"studentName" : "Student2",

 "amountOwed" : 0,

 "tuition" : 500

}

Output should be:

{"studentName" : "Student1",

 "amountOwed" : 200,

 "tuition" : 100

},

{"studentName" : "Student2",

 "amountOwed" : 500,

 "tuition" : 500

}

Is there any way to do this all within one database query?

Maybe using a updateMany?

what is the mongo version you are using? $set (aggregation) is supported in mongo version 4.2+. — niranjan_harpale, Nov 07 '19 at 04:41
https://stackoverflow.com/questions/3974985/update-mongodb-field-using-value-of-another-field this can help you. — niranjan_harpale, Nov 07 '19 at 05:02

score 0 · Answer 1 · answered Nov 07 '19 at 04:44

0

To add values you can use $add expression of Mongo Aggregation Query.

Try using the query:

db.collection.aggregate([
  {
    $project: {
      studentName: 1,
      tuition: 1,
      _id: 0,
      amountOwed: {
        $add: [
          "$amountOwed",
          "$tuition"
        ]
      }
    }
  }
])

answered Nov 07 '19 at 04:44

Mayuri S Kulkarni

771
5
17

He doesn't want to query the collection, he wants to update all the docs. Please read the question. – niranjan_harpale Nov 07 '19 at 04:58
so just add an {$out :"collection"} stage at the end of aggregation query. – matthPen Nov 07 '19 at 07:13

Update all a field in all document based on another field in that document

1 Answers1