How can I speed up this MongoDB query in Mongoose? - node.js

Question

I have a tree-like schema that specifies a collection of parents, and a collection of children.
The collection of children will likely have millions of documents - each of which contains a small amount of data, and a reference to the parent that it belongs to which is stored as a string (perhaps my first mistake).
The collection of parents is much smaller, but may still be in the tens of thousands, and will slowly grow over time. Generally speaking though, a single parent may have as few as 10 children, or as many as 50,000 (possibly more, although somewhat unlikely).
A single child document might look something like this:
{
_id: ObjectId("507f191e810c19729de860ea"),
info: "Here's some info",
timestamp: 1234567890.0,
colour: "Orange",
sequence: 1000,
parent: "12a4567b909c7654d212e45f"
}
Its corresponding parent record (which lives in a separate collection) might look something like this:
{
_id: ObjectId("12a4567b909c7654d212e45f")
info: "Blah",
timestamp: 1234567890.0
}
My query in mongoose (which contains the parent ID in the request) looks like this:
/* GET all children with the specified parent ID */
module.exports.childrenFromParent = function(req, res) {
parentID = req.params.parentID;
childModel.find({
"parentid": parentID
}).sort({"sequence": "asc"}).exec(
function(err, children) {
if (!children) {
sendJSONResponse(res, 404, {
"message": "no children found"
});
return;
} else if (err) {
sendJSONResponse(res, 404, err);
return;
}
sendJSONResponse(res, 200, children);
}
);
};
So basically what's happening is that the query has to search the entire collection of children for any documents that have a parent which matches the provided ID.
I'm currently saving this parent ID as a string in the children collection schema (childModel in the code above), which is probably a bad idea, however, my API is providing the parent ID as a string in the request.
If anyone has any ideas as to how I can either fix my schema or change the query to improve the performance, it would be much appreciated!

Answer 1

Why are you not using .lean() before your exec? Do you really want all of your documents to be Mongoose documents or just simple JSON docs? With lean() you will not get all the extra getters and setters that come with Mongoose document. This could easily shave off at least a second or two from the response time.

Answer 2

Write up from the comments:
You could help speed up and optimize queries by adding an index on the parent field. You can add an (ascending) index by doing the following:
db.collection.createIndex( { parent: 1 } )
You can analyse the benefit of an index by adding .explain("executionStats") to a query. See the docs for more info.
Adding an index on a large collection may take time, you can check the status by running the following query:
db.currentOp(
{
$or: [
{ op: "query", "query.createIndexes": { $exists: true } },
{ op: "insert", ns: /\.system\.indexes\b/ }
]
}
)
Edit: If you are sorting by sequence, you might want to add a compound index for the parent and the sequence.