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I have an aggregate Mongo query that projects some fields and calculates two other ones using $sum. The query works as expected, so I created an unit test for it, and to my surprise the test was failing.

I created a minimal, complete, and verifiable example to test my hypothesis that this was a problem with MongoMock, and it seems to be!

Here is the code:

import mongoengine as mongo
from mongoengine import connect
from mongoengine.queryset import QuerySet

class ValuesList(mongo.EmbeddedDocument):
    updated_value = mongo.DecimalField()


class ValuesHistory(mongo.Document):
    name = mongo.StringField()
    base_value = mongo.DecimalField()
    values_list = mongo.EmbeddedDocumentListField(ValuesList, required=False)
    meta = {
        'collection' : 'values_history'
    }

    def __str__(self):
        return 'name: {}\nbase_value: {}\n'.format(self.name, self.base_value)


def migrate_data(new_collection):
    ValuesHistory.objects.aggregate(
        {'$project': {'name': 1,
                     'base_value': {'$sum': ['$base_value', {'$arrayElemAt': ['$values_list.updated_value', -1]}]}
                      }
        },
        {'$out': "{}".format(new_collection)}
    )


def clear_tables_and_insert_test_data(db):
    db.test.values_history.drop()
    db.test.updated_values.drop()
    ValuesHistory(name='first',
                  base_value=100,
                  values_list=[ValuesList(updated_value=5),
                               ValuesList(updated_value=15)]).save()

def run_aggregate_query_with_db(db):
    new_collection_name = 'updated_values'

    migrate_data(new_collection_name)

    new_group = ValuesHistory.switch_collection(ValuesHistory(), new_collection_name)
    aggregated_values = QuerySet(ValuesHistory, new_group._get_collection()).all()
    for value in aggregated_values:
        print(value)

    db.close()

A quick explanation about the code above.

ValuesHistory is a class that contains a String name, a numeric base_value and a list of values (ValuesList class).

The method clear_tables_and_insert_test_data clears the two tables used in this test and inserts some test data.

The query in migrate_data method creates a new collection (through the $out operator) and the base_value of the newly created collection should be the sum of the current value and the last value in the values_list list. In my case it should be 115 (being 100 the current value and 15 the last value on the list).

If I run the code using a connection to my local MongoDB, like this:

if __name__ == '__main__':
    db = connect('test') # connect to real instance of Mongo
    clear_tables_and_insert_test_data(db)
    run_aggregate_query_with_db(db)

I get 115 as a result, which is exactly what is expected.

If I, instead, use a connection to MongoMock: 

if __name__ == '__main__':
    db = connect('test', host='mongomock://localhost') # Connect to MongoMock instance
    clear_tables_and_insert_test_data(db)
    run_aggregate_query_with_db(db)

I get 100 as result, which is odd! Looks like the $sum operator did not do it's job properly, since the sum of 100 and 15 resulted in 100!

EDIT: I also tried using the $add operator, but the problem remains the same, yielding 100 when it should be 115.

TL;DR;

Question: How should I use $sum (or $add) inside an aggregate pipeline on MongoMock so that it yields the correct value?

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1 回答 1

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所描述的问题确实是 Mongomock 上的一个错误，该错误一直存在到版本 3.14.0。

原始问题发布后，我在 Mongomock 的 github 上打开了一个问题，描述了该问题。在它修复后不久，几天前发布了 3.15.0 版本。我在问题上运行了代码，现在$add和$sum运算符都解决了这个问题！

TL;博士

更新到 Mongomock 3.15.0 足以解决问题。

于 2019-02-22T12:25:09.597 回答