Mongodb: nested field in $group's _id

mongodb

Solution

I found a workaround using `$project`.

db.collection.aggregate([
    { $project: { element_id: '$_id.element_id', count: '$value.count' } },
    { $group: { _id: '$element_id', total: { $sum: '$count' } } }
])

$project Reshapes a document stream by renaming, adding, or removing fields.

http://docs.mongodb.org/manual/reference/aggregation/#_S_project

Problem

Assume we have documents like this in the collection ``` { _id: { element_id: '12345', name: 'foobar' }, value: { count: 1 } } ``` I am using the aggregation framework to do a `$group`, like so ``` db.collection.aggregate([ { $group: { _id: '$_id.element_id', total: { $sum: '$value.count' } } } ]) ``` And got a result of ``` { "result" : [ { "_id" : null, "total" : 1 } ], "ok" : 1 } ``` Notice that the `_id` field in the result is null. From experimentation it seems that $group is not allowing a nested field declaration for its `_id` (e.g. `$_id.element_id`). Why is this? And is there a workaround for it? Thank you.

Original source