I have a collection full of documents with a created_date attribute. I\'d like to send these documents through an aggregation pipeline to do some work on them. Ideally I would
As you already found, you cannot $match on fields that are not in the document (it works exactly the same way that find works) and if you use $project first then you will lose the ability to use indexes.
What you can do instead is combine your efforts as follows:
{
aggregate: 'posts',
pipeline: [
{$match: {
created_date :
{$gte:{$date:'2012-09-01T04:00:00Z'},
$lt: {date:'2012-10-01T04:00:00Z'}
}}
}
},
{$group:
{_id: '0',
totalComments:{$sum:'$comments'}
}
}
]
}
The above only gives you aggregation for September, if you wanted to aggregate for multiple months, you can for example:
{
aggregate: 'posts',
pipeline: [
{$match: {
created_date :
{ $gte:'2012-07-01T04:00:00Z',
$lt: '2012-10-01T04:00:00Z'
}
},
{$project: {
comments: 1,
new_created: {
"yr" : {"$year" : "$created_date"},
"mo" : {"$month" : "$created_date"}
}
}
},
{$group:
{_id: "$new_created",
totalComments:{$sum:'$comments'}
}
}
]
}
and you'll get back something like:
{
"result" : [
{
"_id" : {
"yr" : 2012,
"mo" : 7
},
"totalComments" : 5
},
{
"_id" : {
"yr" : 2012,
"mo" : 8
},
"totalComments" : 19
},
{
"_id" : {
"yr" : 2012,
"mo" : 9
},
"totalComments" : 21
}
],
"ok" : 1
}