Mongodb replace_one() with upsert = true throws duplicate key error

I have found the problem:

in this line of code:

db['dataitemdetails'].replace_one({
    'asset_Id': tdata['asset_id'],
    'period_type': tdata['period_type'],
    'detail_id': tdata['detail_id'],
    'currencycode': tdata['currencycode'],
    'dataitem_Id': tdata['dataitem_id'],
    'period_end': tdata['period_end'],
    'scenario_id': tdata['scenario_id'],
}, tdata, upsert=True)

my filter is wrong.

where asset_Id should have been asset_id, and dataitem_Id should be dataitem_id.

thus no match is found but when inserting a duplicate key error raised.

To answer your first question,

If you don't provide _id field value for upsert=true, MongoDB will generate a new id

And the reason why you are getting the duplicate key error is, you haven't used the period field in your filter condition.

You are telling mongodb to replace one document. Your filter is most likely returning multiple documents.

Use a find with the same filter and figure out a filter that will always return one document. In most cases just using an id should be enough to return one document so long as your schema is layed out correctly.