MongoDb aggregate and group by two fields depending on values - mongodb

Question

I want to aggregate over a collection where a type is given. If the type is foo I want to group by the field author, if the type is bar I want to group by user.
All this should happen in one query.
Example Data:
{
"_id": 1,
"author": {
"someField": "abc",
},
"type": "foo"
}
{
"_id": 2,
"author": {
"someField": "abc",
},
"type": "foo"
}
{
"_id": 3,
"user": {
"someField": "abc",
},
"type": "bar"
}
This user field is only existing if the type is bar.
So basically something like that... tried to express it with an $or.
function () {
var results = db.vote.aggregate( [
{ $or: [ {
{ $match : { type : "foo" } },
{ $group : { _id : "$author", sumAuthor : {$sum : 1} } } },
{ { $match : { type : "bar" } },
{ $group : { _id : "$user", sumUser : {$sum : 1} } }
} ] }
] );
return results;
}
Does someone have a good solution for this?

Answer 1

I think it can be done by
db.c.aggregate([{
$group : {
_id : {
$cond : [{
$eq : [ "$type", "foo"]
}, "author", "user"]
},
sum : {
$sum : 1
}
}
}]);

Answer 2

The solution below can be cleaned up a bit...
For "bar" (note: for "foo", you have to change a bit)
db.vote.aggregate(
{
$project:{
user:{ $ifNull: ["$user", "notbar"]},
type:1
}
},
{
$group:{
_id:{_id:"$user.someField"},
sumUser:{$sum:1}
}
}
)
Also note: In you final answer, anything that is not of type "bar" will have an _id=null

Answer 3

What you want here is the $cond operator, which is a ternary operator returning a specific value where the condition is true or false.
db.vote.aggregate([
{ "$group": {
"_id": null,
"sumUser": {
"$sum": {
"$cond": [ { "$eq": [ "$type", "user" ] }, 1, 0 ]
}
},
"sumAuhtor": {
"$sum": {
"$cond": [ { "$eq": [ "$type", "auhtor" ] }, 1, 0 ]
}
}
}}
])
This basically tests the "type" of the current document and decides whether to pass either 1 or 0 to the $sum operation.
This also avoids errant grouping should the "user" and "author" fields contain the same values as they do in your example. The end result is a single document with the count of both types.