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)

Answer 1

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

1. 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, }

2. 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.

3. 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.