mongodb aggregation framework - generate _id from function - mongodb

Question

Is it possible to have a custom function in the _id field in $group? I couldn't make it work although the documentation seems to indicate that the field can be computed.
For example, let's say I have a set of documents having a number field that ranges 1 to 100. I want to classify the number into several buckets e.g. 1-20, 21-40, etc. Then, I will sum/avg a different field with this bucket identifier. So I am trying to do this:
$group : { _id : bucket("$numberfield") , sum: { $sum: "$otherfield" } }
...where bucket is a function that returns a string e.g. "1-20".
That didn't work.
http://docs.mongodb.org/manual/reference/operator/aggregation/group/#pipe._S_group
For this _id field, you can specify various expressions, including a single field from the documents in the pipeline, a computed value from a previous stage, a document that consists of multiple fields, and other valid expressions, such as constant or subdocument fields.

Answer 1

As at MongoDB 2.4, you cannot implement any custom functions in the Aggregation Framework. If you want to $group by one or more fields, you need to add those either through aggregation operators and expressions or via an explicit update() if you don't want to calculate each time.
Using the Aggregation Framework you can add a computed bucket field in a $project pipeline step with the $cond operator.
Here is an example of calculating ranges based on numberField that can then be used in a $group pipeline for sum/avg/etc:
db.data.aggregate(
{ $project: {
numberfield: 1,
someotherfield: 1,
bucket: {
$cond: [ {$and: [ {$gte: ["$numberfield", 1]}, {$lte: ["$numberfield", 20]} ] }, '1-20', {
$cond: [ {$lt: ["$numberfield", 41]}, '21-40', {
$cond: [ {$lt: ["$numberfield", 61]}, '41-60', {
$cond: [ {$lt: ["$numberfield", 81]}, '61-80', {
$cond: [ {$lt: ["$numberfield", 101]}, '81-100', '100+' ]
}]}]}]}]
}
}},
{ $group: {
_id: "$bucket",
sum: { $sum: "$someotherfield" }
}}
)