Can aggregate functions be used with a variable in the field name? - mongodb

Question

I have some documents that look like this:
{
(...stuff...)
something: {
first: {
0: 3,
1: 5,
2: 2
},
second: {
0: 1,
1: 9,
2: 7
}
}
}
For the sake of simplicity in this question, I'll assume that my $match only hits this one document. What I'd like to do is, in an aggregate command, add up the 0s, and add up the 1s, and add up the 2s, so that I can produce something like this:
something.0: 4 (something.first.0 + something.second.0)
something.1: 14 (something.first.1 + something.second.1)
something.2: 9 (something.first.2 + something.second.2)
Is this something that can be done, or do I need to change my document schema to rearrange the nested documents such that all the 0s are together, etc?

Answer 1

If you know the names of all the arrays you want to sum, you can use the Aggregation Framework in MongoDB 2.2+ and a projection with the $add operator:
db.something.aggregate(
{ $project: {
something: {
0: { $add: [ "$something.first.0", "$something.second.0" ] },
1: { $add: [ "$something.first.1", "$something.second.1" ] },
2: { $add: [ "$something.first.2", "$something.second.2" ] }
}
}}
)
Sample output:
{
"result" : [
{
"_id" : ObjectId("517ae4914bc9ade96ca6402d"),
"something" : {
"0" : 4,
"1" : 14,
"2" : 9
}
}
],
"ok" : 1
}
If you don't know the names (i.e. you are using trying to add the 0th element of every array element in a document) you will need to use MapReduce or implement the logic in your application code instead.