Transform a mongoDB cursor to JSON - node.js

Question

I have a mongoDB database set up with a express server.
I try to access a object which is inside an array in a document.
I have this route :
app.get("/api/" + BLOGS_COLLECTION + "/:blogId" + "/post" + "/:postId", function (req, res) {
db.collection(BLOGS_COLLECTION).aggregate([
{ $match: { _id: req.params.postId } }, {
$project: {
posts: {
$filter: {
input: "$posts",
as: "posts",
cond: { $eq: ["$$posts._id", req.params.postId] }
}
}
}
}].toArray((err, doc) => {
console.log(doc)
if (err) {
handleError(res, err.message, "Failed to get post");
} else {
res.status(200).send(doc);
}
}));
});
But it returns an error :
[{(intermediate value)},{(intermediate value)}].toArray is not a function
If I do not use the toArray but a plain function, it can't construct to json to send it to the front.
I do not use Mongoose.
If the code above can't work, how can I query only the object I want inside the array ?
P.S: this is how my database is made :
_id:60336bcc961785586471938b
title:"<p>ttttttttttttttttttttttttttt</p>"
description:"<p>tttttttttttttttttttttttttt</p>"
authorId:"60336bb5961785586471938a"
createDate:2021-02-22T08:31:08.889+00:00
posts:
0:
_id:"53394732-d60b-c869-1fed-1fb82c03780f"
title:"<p>iiiiiiiiiiiiiiiiiiiiiiiiiiiii</p>"
content:"<p>iiiiiiiiiiiiiiiiiiii</p>"
date:"2021-02-22T08:31:14.911Z"

Answer 1

You need to call toArray on the cursor reference you get back from the aggregation and not on the aggregation array:
db.collection(BLOGS_COLLECTION).aggregate([
{ $match: { _id: req.params.postId } },
{
$project: {
posts: {
$filter: {
input: "$posts",
as: "posts",
cond: { $eq: ["$$posts._id", req.params.postId] }
}
}
}
}
],
(err, cursor) => {
if (err) {
handleError(res, err.message, "Failed to get post");
} else {
cursor.toArray((error, documents) => {
if (error) { return handleError(res, error.message, "Failed to get post"); }
console.log(documents)
res.status(200).send(documents);
});
}
});
MongoDB Aggregation Pipeline

Answer 2

Your stages between square prackets should look like this:
[
{ $match: { _id: ObjectId( req.params.blogId) } },
{
$project: {
posts: {
$filter: {
input: "$posts",
as: "posts",
cond: { $eq: ["$$posts._id", ObjectId(req.params.postId)] }
}
}
}
},
{$unwind : "$posts" },
{$replaceRoot :{ newRoot:"$posts"}
]
After filtering and projecting, unwind array objects and then simply replace the root. And of course, follow the instructions from the previous answer about the usage of toArray on aggregate cursor result.