Query multiple properties in at the same time getting an overall average and an array - mongodb

Question

Given the following data, I'm trying to get an average of all their ages, at the same time I want to return an array of their names. Ideally, I want to do this in just one query but can't seem to figure it out.
Data:
users:[
{user:{
id: 1,
name: “Bob”,
age: 23
}},
{user:{
id: 1,
name: “Susan”,
age: 32
}},
{user:{
id: 2,
name: “Jeff”,
age: 45
}
}]
Query:
var dbmatch = db.users.aggregate([
{$match: {"id" : 1}},
{$group: {_id: null, avg_age: { $avg: "$age" }}},
{$group: {_id : { name: "$name"}}}
)]
Running the above groups one at a time outputs the results I expect, either an _id of null and an average of 27.5, or an array of the names.
When I combine them as you see above using a comma, I get:
Issue Generated Code:
[ { _id: {name: null } } ]
Expected Generated Code:
[
{name:"Bob"},
{name:"Susan"},
avg_age: 27.5
]
Any help would be greatly appreciated!

Answer 1

Not sure if this is exactly what you want, but this query
db.users.aggregate([
{
$match: {
id: 1
}
},
{
$group: {
_id: "$id",
avg_age: {
$avg: "$age"
},
names: {
$push: {
name: "$name"
}
}
}
},
{
$project: {
_id: 0
}
}
])
Results in this result:
[
{
"avg_age": 27.5,
"names": [
{
"name": "Bob"
},
{
"name": "Susan"
}
]
}
]
This will duplicate names, so if there are two documents with the name Bob, it will be two times in the array. If you don't want duplicates, change $push to $addToSet.
Also, if you want names to be just an array of names instead of objects, change names query to
names: {
$push: "$name"
}
This will result in
[
{
"avg_age": 27.5,
"names": ["Bob", "Susan"]
}
]
Hope it helps,
Tomas :)

Answer 2

You can use $facet aggregation to run the multiple queries at once
db.collection.aggregate([
{ "$facet": {
"firstQuery": [
{ "$match": { "id": 1 }},
{ "$group": {
"_id": null,
"avg_age": { "$avg": "$age" }
}}
],
"secondQuery": [
{ "$match": { "id": 1 }},
{ "$group": { "_id": "$name" }}
]
}}
])