Mongoose - finding subdocuments by criteria - node.js

Question

I've just got stuck with this problem. I've got two Mongoose schemas:
var childrenSchema = mongoose.Schema({
name: {
type: String
},
age: {
type: Number,
min: 0
}
});
var parentSchema = mongoose.Schema({
name : {
type: String
},
children: [childrenSchema]
});
Question is, how to fetch all subdocuments (in this case, childrenSchema objects) from every parent document? Let's suppose I have some data:
var parents = [
{ name: "John Smith",
children: [
{ name: "Peter", age: 2 }, { name: "Margaret", age: 20 }
]},
{ name: "Another Smith",
children: [
{ name: "Martha", age: 10 }, { name: "John", age: 22 }
]}
];
I would like to retrieve - in a single query - all children older than 18. Is it possible? Every answer will be appreciated, thanks!

Answer 1

You can use $elemMatch as a query-projection operator in the most recent MongoDB versions. From the mongo shell:
db.parents.find(
{'children.age': {$gte: 18}},
{children:{$elemMatch:{age: {$gte: 18}}}})
This filters younger children's documents out of the children array:
{ "_id" : ..., "children" : [ { "name" : "Margaret", "age" : 20 } ] }
{ "_id" : ..., "children" : [ { "name" : "John", "age" : 22 } ] }
As you can see, children are still grouped inside their parent documents. MongoDB queries return documents from collections. You can use the aggregation framework's $unwind method to split them into separate documents:
> db.parents.aggregate({
$match: {'children.age': {$gte: 18}}
}, {
$unwind: '$children'
}, {
$match: {'children.age': {$gte: 18}}
}, {
$project: {
name: '$children.name',
age:'$children.age'
}
})
{
"result" : [
{
"_id" : ObjectId("51a7bf04dacca8ba98434eb5"),
"name" : "Margaret",
"age" : 20
},
{
"_id" : ObjectId("51a7bf04dacca8ba98434eb6"),
"name" : "John",
"age" : 22
}
],
"ok" : 1
}
I repeat the $match clause for performance: the first time through it eliminates parents with no children at least 18 years old, so the $unwind only considers useful documents. The second $match removes $unwind output that doesn't match, and the $project hoists children's info from subdocuments to the top level.

Answer 2

In Mongoose, you can also use the elegant .populate() function like this:
parents
.find({})
.populate({
path: 'children',
match: { age: { $gte: 18 }},
select: 'name age -_id'
})
.exec()

Answer 3

A. Jesse Jiryu Davis's response works like a charm, however for later versions of Mongoose (Mongoose 5.x) we get the error:
Mongoose 5.x disallows passing a spread of operators to Model.aggregate(). Instead of Model.aggregate({ $match }, { $skip }), do Model.aggregate([{ $match }, { $skip }])
So the code would simply now be:
> db.parents.aggregate([{
$match: {'children.age': {$gte: 18}}
}, {
$unwind: '$children'
}, {
$match: {'children.age': {$gte: 18}}
}, {
$project: {
name: '$children.name',
age:'$children.age'
}
}])
{
"result" : [
{
"_id" : ObjectId("51a7bf04dacca8ba98434eb5"),
"name" : "Margaret",
"age" : 20
},
{
"_id" : ObjectId("51a7bf04dacca8ba98434eb6"),
"name" : "John",
"age" : 22
}
],
"ok" : 1
}
(note the array brackets around the queries)
Hope this helps someone!