Fix BaseQuerySet#sum and BaseQuerySet#average for fields that specify a db_field

[wojcikstefan]

Fixes #1415

@lafrech @zeezdev take a look please

[wojcikstefan]

@yograterol just curious, is the "In Progress" label assignment an automated hook of some sort? Does anybody find it useful (cc @lafrech)?

[lafrech]

I have no idea what this InProgress tag is for and how it is generated.

[zeezdev]

It works, but to sum it is necessary to specify the names of the models instead of the field names in the database that is not compatible with old behavior of aggregate_sum.

[wojcikstefan]

Hm, using db fields directly seems like a invalid behavior. That being said, I understand aggregate_sum worked this way and that we should have added a warning in the changelog stating that the new sum doesn't follow the same logic. As far as I remember though, current behavior is consistent with the way the old sum/average worked.

[lafrech]

As far as I remember though, current behavior is consistent with the way the old sum/average worked.

Then you couldn't do much better. It would have been complicated to both fit old sum/avg behaviour and keep compatibility with aggregate_sum/avg, if both behaved differently.

The aggregate_ versions were not advertised in the docs, so from the docs perspective, nothing is broken. Yet, you may want to add a note to the docs:

https://github.com/MongoEngine/mongoengine/blob/master/docs/guide/querying.rst#further-aggregation

to specify that it works with the model names, and to warn about the change in latest MongoEngine:

• now using aggreg framework
• removal of aggregation_*

for anyone being affected by the change.