Adding object to array in array in MongoDB - arrays

Question

I have this data structure (classes with comments) and I want to add a 1-level deep reply. This means that I'd like to add another object on a "comentarios" element.
How can I achieve this with mongo?
This means: Match cursada (db) id, match clase (first array) id, match comment (second array) id, then add a new element there.
db.cursada.find({"_id": ObjectId("55444f56e5e154f7638b456a")}).pretty()
{
"_id" : ObjectId("55444f56e5e154f7638b456a"),
"clases" : [
{
"_id" : "554e7f2fe5e154797d8b4578",
"titulo" : "qewewqewq"
},
{
"_id" : "554e8be0e5e154dc698b4582",
"titulo" : "la mejor clase"
},
{
"_id" : "554eb90de5e154dd698b458b",
"comentarios" : [
{
"_id" : ObjectId("55a021afe5e154cf098b4567"),
"nombreUsuario" : "nombre",
"texto" : "432432423"
}
],
"titulo" : "Peeling - cosa"
},
{
"_id" : "554e91a0e5e154797d8b4587",
"titulo" : "fdsfdsa"
},
{
"_id" : "554f8f50e5e154dd698b458f",
"titulo" : "clase2"
},
{
"_id" : "554f99dae5e154797d8b45a7",
"titulo" : "con profesor"
},
{
"_id" : "554fa4a0e5e154797d8b45c4",
"titulo" : "profesor nombre nombre"
},
{
"_id" : "5557b37be5e154e07f8b4567",
"titulo" : "Dermatologia I"
},
{
"_id" : "5557c701e5e154066d8b456c",
"titulo" : "Acido hialuronico"
}
],
"curso" : "552fa5f1e5e1542e628b4567",
"fechaFin" : "2015-05-22T03:00:00.000Z",
"fechaIni" : "2015-05-08T03:00:00.000Z",
"titulo" : "cremotas"
}
Getting this result:
{
"_id" : ObjectId("55444f56e5e154f7638b456a"),
"clases" : [
{
"_id" : ObjectId("554eb90de5e154dd698b458b"),
"comentarios" : [
{
"_id" : ObjectId("55a021afe5e154cf098b4567"),
"nombreUsuario" : "nombre",
"texto" : "432432423",
----------------HERE
"replies": [
{ "_id": ....,
"user": ....,
"text":....,
"date":....
}]
----------------HERE
}
],
"titulo" : "Peeling - cosa"
},
]
}

Answer 1

One twisted example!
Luckily it presents no problem to MongoDB and its $elemMatch query operator:
Model.update({
_id: "55444f56e5e154f7638b456a",
classes: {
$elemMatch: {
_id: "554eb90de5e154dd698b458b",
}
}
}, {
$push: {
'classes.$.comentarios': {
nombreUsuario: 'New comment nombreUsuario',
texto: '111242515'
}
}
}, function(err, count) {
console.log('Updated ' + count + ' document');
});
What happens here?
First, we're specyfing specific course ("cursada") to update:
_id: "55444f56e5e154f7638b456a"
Then, using $elemMatch operator, we're restricting result of the query to contain only one class:
classes: {
$elemMatch: {
_id: "554eb90de5e154dd698b458b",
}
}
Now, having found specific class to update, we can finally add new comment to it. Here's our update query:
$push: {
'classes.$.comentarios': {
nombreUsuario: 'New comment nombreUsuario',
texto: '111242515'
}
}
$push operator tells MongoDB to add new comment to specified array of comments.
'classes.$.comentarios' specifies which nested comentarios array to update, using $ positional operator.
Finally, and this part should be self-explanatory: full object of new comment to add to the specified class.
It's also worth mentioning that if you're running into such deeply nested structure, perhaps it's worth thinking about spreading courses, classes, possibly even comments over separate MongoDB collections.
When nested documents are the way to go and when it's better to create separate collections can be actually tricky question to answer - here's nice presentation discussing this issue.