Hello everyone,

I have few questions about new aggregation framework.

Question 1. Does non-existence of a field considered as $min value in a
$group?

For e.g. In the following script, min_metric2 is not printed because
at-least one document doesn't have it. How do I find the min value among
the remaining documents?

db.metric.remove({});
db.metric.save({year: 2012, month: 1, metrics: {metric1: 30, metric2: 80}});
db.metric.save({year: 2012, month: 2, metrics: {metric1: 50, metric2: 20}});
db.metric.save({year: 2012, month: 3, metrics: {metric1: 10}});
db.metric.save({year: 2012, month: 4, metrics: {metric1: 20, metric2: 40}});
db.metric.save({year: 2012, month: 5, metrics: {metric1: 40, metric2: 60}});

var result = db.metric.aggregate
(
[
{
$group: {
_id: {year: "$year"},
min_metric1: {$min: "$metrics.metric1"},
min_metric2: {$min: "$metrics.metric2"},
max_metric1: {$max: "$metrics.metric1"},
max_metric2: {$max: "$metrics.metric2"},

printjson(result);

================================================

{
        "result" : [
                {
                        "_id" : {
                                "year" : 2012
                        },
                        "min_metric1" : 10,
                        "max_metric1" : 50,
                        "max_metric2" : 80
                }
        ],
        "ok" : 1

Question 3: Is it possible to define a field conditionally (based on a
computed value) in $project?

Thank you.