Update a field in a deeply nested document in Mongodb - mongodb

Question

The MongoDB document structure in question looks like this :
{
"_id": ObjectId("54247a68fab6b6775d000062"),
"owner": "1",
"version": "Version 1",
"name": "Test20",
"u_at": ISODate("2014-09-25T20:26:16.140Z"),
"c_at": ISODate("2014-09-25T20:26:16.140Z"),
"canvases": [
{
"_id": ObjectId("54247a68fab6b6775d000063"),
"nodes": [
{
"_id": ObjectId("54247a68fab6b6775d000060"),
"filePathTemplate": "LETSDOEMAIL"
},
{
"_id": ObjectId("54247a68fab6b6775d000061"),
"filePathTemplate": "LETSDOFACEBOOK"
}
]
}
]
}
I am struggling primarily with two things:
Searching for a specific node and get only the node back in result. Following is the query I am currently using (after browsing all related SO questions):
db.getCollection("coll").find({_id: ObjectId("54247a68fab6b6775d000062")}, {canvases:{$elemMatch:{nodes:{$elemMatch:{_id: ObjectId("54247a68fab6b6775d000060")}}}}})
But this gives back the canvas, containing the node searched for, instead of node.
{
"_id": ObjectId("54247a68fab6b6775d000062"),
"canvases": [
{
"_id": ObjectId("54247a68fab6b6775d000063"),
"nodes": [
{
"_id": ObjectId("54247a68fab6b6775d000060"),
"filePathTemplate": "LETSDOEMAIL"
},
{
"_id": ObjectId("54247a68fab6b6775d000061"),
"filePathTemplate": "LETSDOFACEBOOK"
}
]
}
]
}
As a result of above mentioned issue, updating a field in a node document is also a problem. This is the query I have got from other SO questions but to no avail:
db.getCollection("coll").update({canvases: {$elemMatch:{nodes:{$elemMatch:{_id: ObjectId("54247a68fab6b6775d000060")}}}}}, {$set: {"canvases.$.nodes.$.filePathTemplate": "21"}})
Any help would be appreciated.

Answer 1

Question 1:
Only the first $elemMatch in the second parameter of .find(arg1, arg2) is effective to position element of array, that is, only one element of canvases can be positioned, excluding nodes'. So .find() is improper to do this kind of task in my opinion.
But I think you can reach the target by following method:
db.coll.aggregate([ {
$match : {
_id : ObjectId("54247a68fab6b6775d000062")
}
}, {
$redact : {
$cond : [ {
$or : [ {
$gt : [ "$canvases", [] ]
}, {
$gt : [ "$nodes", [] ]
}, {
$eq : [ "$_id", ObjectId("54247a68fab6b6775d000060") ]
} ]
}, "$$DESCEND", "$$PRUNE" ]
}
} ]);
Question 2:
Positional operator $ can not use in nested array;
So when you use $ after an array, all the rest of path must be "solid"; it means if there are nested array behind $, you must explicitly write out its index to point out which element you want to update.
Suppose all _id in all document level of this collection is unique.
Following code for your reference:
var searchKey = ObjectId("54247a68fab6b6775d000060");
var filePathTemplate = "21";
db.getCollection("coll").find({
canvases : {
$elemMatch : {
nodes : {
$elemMatch : {
_id : searchKey
}
}
}
}
}, {
"canvases.$" : 1
}).forEach(function(doc) {
// We need to find out the index of that sub-document which is required to
// updated in nodes, because nodes is probably very big.
var i = 0;
var nodes = doc.canvases[0].nodes; // Only one element in canvases from abover query.
for (; i < nodes.length && nodes[i]._id.str != searchKey.str; ++i);
var key = "canvases.$.nodes." + i + ".filePathTemplate"; // Only one "$" can be used.
var updateNodePart = {};
updateNodePart[key] = filePathTemplate;
db.getCollection("coll").update({
canvases : {
$elemMatch : {
nodes : {
$elemMatch : {
_id : searchKey
}
}
}
}
}, {
$set : updateNodePart
});
});