mongodb: how to create a calculated field using python (pymongo)?

Question

Is there a way to create a calculated field in mongodb using pymongo?

example = {
    "field1": 1,
    "field2": 2,
    "calculated_field": "field1" + "field2"
}

The calculated field must always keep the formula, if "field1" will later be modified, the result must update.

I have read mongodb documentation and I can see it can be done with aggregation pipeline but pymongo's documentation is not really clear on this procedure.

Edit:

I am trying, at the moment, to insert a new field as below but the field is not added.

    pipeline = [
    {
        "$addFields": {
            "calculated_field": {"$sum": ["field1", "field2"]}
        }
    }
]

dbCollection = database["col"]

dbCollection.aggregate(pipeline)

I have tried like in the edit and other examples from pymongo documentation but nothing seems to work. I'm a little lost and I think I'm missing something. — Sergiu Elmi
– Sergiu Elmi, Commented Aug 3, 2020 at 11:02

Lauren Schaefer · Accepted Answer · 2020-08-03 12:02:01Z

1

You have a couple of different options based on what your goals are.

You can use the aggregation pipeline to calculate the sum:

 [
     {
         '$addFields': {
             'total': {
                 '$sum': [
                     '$field1', '$field2'
                 ]
             }
         }
     }
 ]

The result will be a document that has the total field. Keep in mind that this will not store the total in your database.

  {
      field1: 5,
      field2: 3,
      total: 8,
  }

You could create a Change Stream to monitor field1 and field2 to check for changes. When changes are made, you could automatically update the total that is stored in your database. See https://developer.mongodb.com/quickstart/python-change-streams for more information on how to create Change Streams in Python.
If your database is stored in MongoDB Atlas (MongoDB's fully managed database-as-a-service), you can use a Trigger to monitor field1 and field2 for changes. Triggers are built on the same concepts as Change Streams. Triggers are a bit simpler since you don't have worry about hosting and managing the Change Stream yourself. See https://docs.atlas.mongodb.com/triggers/ for more information on Triggers.

answered Aug 3, 2020 at 12:02

Lauren Schaefer

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3 Comments

Sergiu Elmi Over a year ago

I have tried the exact pipeline you described but the field is not added. From python I have tried both "dbCollection.aggregate(pipeline)" and "database.command('aggregate', "collection", pipeline=pipeline, explain=True)".

Sergiu Elmi Over a year ago

anyway.. I have used another approach but thank you for the help and good read!

Lauren Schaefer Over a year ago

Glad you were able to get it working! To clarify further - the aggregation pipeline does not save the results to the database unless you add a stage to specifically do so like $out (docs.mongodb.com/manual/reference/operator/aggregation/out). If you don't use $out, the results of the pipeline will be returned to you so you can see them and work with them.

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