aggregate_sum/average + unit tests

[wojcikstefan]

Closes #569

@MRigal here's the implementation of sum & average using the aggregation framework. I haven't tested it in a sharded environment, but if it works, I believe it could fully replace the slower, map-reduce-based BaseQuerySet.sum and BaseQuerySet.average.

[coveralls]

Coverage decreased (-0.02%) to 91.17% when pulling 0869bdf on elasticsales:aggregate-sum-and-avg into 6d3bc43 on MongoEngine:master.

[landscape-bot]

Repository health decreased by 0.04% when pulling 0869bdf on elasticsales:aggregate-sum-and-avg into 6d3bc43 on MongoEngine:master.

• 2 new problems were found (including 0 errors and 2 code smells).
• No problems were fixed.

[wojcikstefan]

Thanks @thedrow, the latest commit fixes it.

[landscape-bot]

Repository health decreased by 0.05% when pulling ce8f960 on elasticsales:aggregate-sum-and-avg into 6d3bc43 on MongoEngine:master.

• 2 new problems were found (including 0 errors and 2 code smells).
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[MRigal]

Thanks @wojcikstefan !
You will find me very picky, but for readability and to minimise the "PyMongo3-related code conditionality", could you eventually rewrite the followings:

   if IS_PYMONGO_3:
        result = list(result)
        if result:
            return result[0]['total']
    else:
        if result['result']:
            return result['result'][0]['total']
    return 0

to:

   if IS_PYMONGO_3:
        result = list(result)
    else:
        result = result.get('result')
    if result:
        return result[0]['total']
    return 0

Thanks !

[MRigal]

To all:
should we also add some DeprecationWarning to the classical sum() and average() methods?

[MRigal]

One more point before merge: @wojcikstefan please add this to the changelog !

[wojcikstefan]

@MRigal thanks - glad you pay attention! I added a changelog entry and simplified the code.

Not sure about deprecating sum and average. Ideally, we would just drop the aggregate_ prefix from the methods I implemented here. In such case, the methods themselves wouldn't be deprecated, but their original contents would.

[landscape-bot]

Repository health decreased by 0.04% when pulling 4e0b54d on elasticsales:aggregate-sum-and-avg into 6d3bc43 on MongoEngine:master.

• 2 new problems were found (including 0 errors and 2 code smells).
• No problems were fixed.

[MRigal]

@thedrow @DavidBord why the fuck did Travis didn't start a build Job here???!?!? I have no clue how to start one now...

[MRigal]

@wojcikstefan I would have eventually solved the conflict by hand, but since the tests didn't started, could you please rebase the branch and push, with the hope it will launch a test run on travis...

[wojcikstefan]

@MRigal I rebased against the latest master. It seems to have triggered a Travis build.

[landscape-bot]

Repository health decreased by 0.04% when pulling b7ef82c on elasticsales:aggregate-sum-and-avg into 4d5200c on MongoEngine:master.

• 2 new problems were found (including 0 errors and 2 code smells).
• No problems were fixed.

[MRigal]

Thanks @wojcikstefan !

[lafrech]

Thanks @wojcikstefan for this contribution. I found those methods by accident while looking in the code. They are not documented in the doc.

@MongoEngine/mongoengine, what would make more sense? Adding a note to the docs? Replace old sum() and average() and drop old map/reduce methods?

Are there cases not covered by those aggregate_* methods? (Old MongoDB, old pymongo, specific configuration,...)

[wojcikstefan]

I think it would make sense to replace the original methods with the new ones. I believe it's supported by all versions of MongoDB >= 2.2 when the aggregation framework was introduced. Also, the less raw JS code in MongoEngine, the better IMHO :)

Curious what @thedrow thinks about this!