Develop Reference

How to insert an Array in Node js and MongoDB

Well, I am stuck since a long time on how to treat properly an array in Node js in order to persist it in my mongobd database.
I tried a lot of stuff, i am feeling close to success, but still stuck with it with the var filmDataSchema and the var film syntax.
I really need a little help, please:

Are you using mongoose? it's an elegant mongodb object modeling for node.js and has a really nice interface to handle arrays so let's say you have a student model with grades array and you want to add a new grade, you can use the '$push' (if) or $addToSet operators like so
StudentModel.update(
{ _id: student._id },
{ $addToSet: { grades: grade } }
);
StudentModel.update(
{ _id: student._id },
{ $push: { grades: grade } }
);
use $addToSet if you want only unique items to be pushed into array. use $push if you just want to add the object to array whether or not the object is already there
more about mongoose can be found here https://mongoosejs.com/

MongoDB is going back to matching among all documents after $group stage

So I have a collection looking like this:
[
{"url":"website.com/test", "links":[ {"url": "www.something.fr/page.html","scoreDiff": 0.44} ], "justUpdated": true, "score": 0.91},
{"url":"domain.com/", "links":[], "justUpdated": true, "score": 0.81},
{"url":"www.something.fr/page.html", "links":[], "justUpdated": false, "score": 0.42},
]
The goal here is to get the third document, because in one of the documents where "justUpdated" equals true (the first one here), there is its url as a value in one of the "links" array elements.
To achieve that, I tried:
To find all the documents with "justUpdated" equals to true, then in NodeJS concatenate all the urls in their "links" arrays (let's call this array urlsOfInterest). And finally do another query to find all the documents where the url is in urlsOfInterest.
The problem is that it takes some time to do the first query then process the result and do the second query.
So I thought maybe I could do it all at once in an aggregate query. I use $group (with $cond to check if justUpdated equals true) to get all the arrays of "links" in one new variable named urlsOfInterest. For now this is an array of arrays of object so I then use $project with $reduce to have all these {url: "...", score: X} objects as one big array. Finally I use $project and $map to only have the url as the score value doesn't interest me here.
So I get an output looking like this:
_id:"urlsOfInterest",
urlsOfInterest: ["www.something.fr/page.html"]
Which is pretty great but I am stuck because now I just need to get the documents where url is in this variable named urlsOfInterest except I can't because all my documents have "disappeared" after the $group stage.
Please help me to find a way to perform this final query :) Or if this isn't the right way to do this, please point me in the right direction !
PS: the real goal here would be to update for all the documents where justUpdated equals true, every scoreDiff values in their links array. For our exemple, we do abs(0.91 - 0.42) = 0.49 so we replace our scoreDiff value of 0.44 by 0.49 (0.91 being the score of the document where justUpdated equals true and 0.42 the score of the document where url equals www.something.fr/page.html, explaining why I need to fetch this last document.) I don't believe there could be a way of doing all of this at once but if there is, please tell me !

You can use $lookup to get all matching documents in an array:
db.collection.aggregate([
{
"$match": {
"justUpdated": true
}
},
{
"$lookup": {
"from": "collection",
"localField": "links.url",
"foreignField": "url",
"as": "result"
}
},
{
"$match": {
"result": {
$gt: []
}
}
}
])
Then either $unwind and $replaceRoot the results array to get the documents as a cursor and do the math on the application level or do the calculations in the same pipeline, e.g. with $reduce
The "PS: the real goal" is not quite clear as it is based on a particular example but if you play a little bit with it in the playground I am sure you can calculate the numbers as per your requirements.

Not able to use sort() inside of map function

The documents in my couchdb look like this:
{
docType: "event",
tags: ["bb","aa","cc"],
...
}
I would like my couchdb view function, to emit a sorted array of tags, so I tried:
function(doc) {
if (doc.docType == 'event') {
if (doc.tags.length > 0) {
emit(doc.tags.sort(), doc._id);
}
}
}
But this (.sort()) is not working as expected: The result shows only documents, where the tags array has only one entity (tags.length = 1).

I found the answer by the help of the couchDB Slack channel. sort() wants to change the positions of the elements in the original array, which is not intended by the view function. You can see exceptions of the javascript engine for each array, to be sorted and which has more than one entries. Need to switch the log level of couchdb to "debug" before.
Using the spread operator to create a new array solved the problem:
emit([...doc.tags].sort(), doc._id);

Update field value in nested array object

I have objects that pretty much look like this:
{
...
someArray: [
{
id: 1548,
amount: 153,
done: 0
}
]
...
}
As these objects can become quite large, I can't just use set everytime I'm updating them, as sending 100kB everytime I need to update the document isn't an option.
In order to solve this, I decided to use update with what's called the "dots notation", example usage being :
update({
'a.b.c': true
})
Source: Difference between set with {merge: true} and update
So I decided to give it a try and it worked like a charm for "normal" nested fields, but I can't find how I can do this for objects that are nested inside arrays.
What I tried was this:
update({
'a.someArray.0.done': 153
})
update({
'a.someArray[0].done': 153
})
But both of these just erased the object and replaced it by the patch, meaning that the dots notation wasn't recognized properly.
How can I solve this? Is there a solution for this kind of approach or should I just refactor it using a subcollection?

I believe this could be what you are looking for:
var washingtonRef = db.collection("cities").doc("DC");
// Atomically add a new region to the "regions" array field.
washingtonRef.update({
regions: firebase.firestore.FieldValue.arrayUnion("greater_virginia")
});
https://firebase.google.com/docs/firestore/manage-data/add-data#update_elements_in_an_array

Updating a Nested Array with MongoDB

I am trying to update a value in the nested array but can't get it to work.
My object is like this
{
"_id": {
"$oid": "1"
},
"array1": [
{
"_id": "12",
"array2": [
{
"_id": "123",
"answeredBy": [], // need to push "success"
},
{
"_id": "124",
"answeredBy": [],
}
],
}
]
}
I need to push a value to "answeredBy" array.
In the below example, I tried pushing "success" string to the "answeredBy" array of the "123 _id" object but it does not work.
callback = function(err,value){
if(err){
res.send(err);
}else{
res.send(value);
}
};
conditions = {
"_id": 1,
"array1._id": 12,
"array2._id": 123
};
updates = {
$push: {
"array2.$.answeredBy": "success"
}
};
options = {
upsert: true
};
Model.update(conditions, updates, options, callback);
I found this link, but its answer only says I should use object like structure instead of array's. This cannot be applied in my situation. I really need my object to be nested in arrays
It would be great if you can help me out here. I've been spending hours to figure this out.
Thank you in advance!

General Scope and Explanation
There are a few things wrong with what you are doing here. Firstly your query conditions. You are referring to several _id values where you should not need to, and at least one of which is not on the top level.
In order to get into a "nested" value and also presuming that _id value is unique and would not appear in any other document, you query form should be like this:
Model.update(
{ "array1.array2._id": "123" },
{ "$push": { "array1.0.array2.$.answeredBy": "success" } },
function(err,numAffected) {
// something with the result in here
}
);
Now that would actually work, but really it is only a fluke that it does as there are very good reasons why it should not work for you.
The important reading is in the official documentation for the positional $ operator under the subject of "Nested Arrays". What this says is:
The positional $ operator cannot be used for queries which traverse more than one array, such as queries that traverse arrays nested within other arrays, because the replacement for the $ placeholder is a single value
Specifically what that means is the element that will be matched and returned in the positional placeholder is the value of the index from the first matching array. This means in your case the matching index on the "top" level array.
So if you look at the query notation as shown, we have "hardcoded" the first ( or 0 index ) position in the top level array, and it just so happens that the matching element within "array2" is also the zero index entry.
To demonstrate this you can change the matching _id value to "124" and the result will $push an new entry onto the element with _id "123" as they are both in the zero index entry of "array1" and that is the value returned to the placeholder.
So that is the general problem with nesting arrays. You could remove one of the levels and you would still be able to $push to the correct element in your "top" array, but there would still be multiple levels.
Try to avoid nesting arrays as you will run into update problems as is shown.
The general case is to "flatten" the things you "think" are "levels" and actually make theses "attributes" on the final detail items. For example, the "flattened" form of the structure in the question should be something like:
{
"answers": [
{ "by": "success", "type2": "123", "type1": "12" }
]
}
Or even when accepting the inner array is $push only, and never updated:
{
"array": [
{ "type1": "12", "type2": "123", "answeredBy": ["success"] },
{ "type1": "12", "type2": "124", "answeredBy": [] }
]
}
Which both lend themselves to atomic updates within the scope of the positional $ operator
MongoDB 3.6 and Above
From MongoDB 3.6 there are new features available to work with nested arrays. This uses the positional filtered $[<identifier>] syntax in order to match the specific elements and apply different conditions through arrayFilters in the update statement:
Model.update(
{
"_id": 1,
"array1": {
"$elemMatch": {
"_id": "12","array2._id": "123"
}
}
},
{
"$push": { "array1.$[outer].array2.$[inner].answeredBy": "success" }
},
{
"arrayFilters": [{ "outer._id": "12" },{ "inner._id": "123" }]
}
)
The "arrayFilters" as passed to the options for .update() or even
.updateOne(), .updateMany(), .findOneAndUpdate() or .bulkWrite() method specifies the conditions to match on the identifier given in the update statement. Any elements that match the condition given will be updated.
Because the structure is "nested", we actually use "multiple filters" as is specified with an "array" of filter definitions as shown. The marked "identifier" is used in matching against the positional filtered $[<identifier>] syntax actually used in the update block of the statement. In this case inner and outer are the identifiers used for each condition as specified with the nested chain.
This new expansion makes the update of nested array content possible, but it does not really help with the practicality of "querying" such data, so the same caveats apply as explained earlier.
You typically really "mean" to express as "attributes", even if your brain initially thinks "nesting", it's just usually a reaction to how you believe the "previous relational parts" come together. In reality you really need more denormalization.
Also see How to Update Multiple Array Elements in mongodb, since these new update operators actually match and update "multiple array elements" rather than just the first, which has been the previous action of positional updates.
NOTE Somewhat ironically, since this is specified in the "options" argument for .update() and like methods, the syntax is generally compatible with all recent release driver versions.
However this is not true of the mongo shell, since the way the method is implemented there ( "ironically for backward compatibility" ) the arrayFilters argument is not recognized and removed by an internal method that parses the options in order to deliver "backward compatibility" with prior MongoDB server versions and a "legacy" .update() API call syntax.
So if you want to use the command in the mongo shell or other "shell based" products ( notably Robo 3T ) you need a latest version from either the development branch or production release as of 3.6 or greater.
See also positional all $[] which also updates "multiple array elements" but without applying to specified conditions and applies to all elements in the array where that is the desired action.

I know this is a very old question, but I just struggled with this problem myself, and found, what I believe to be, a better answer.
A way to solve this problem is to use Sub-Documents. This is done by nesting schemas within your schemas
MainSchema = new mongoose.Schema({
array1: [Array1Schema]
})
Array1Schema = new mongoose.Schema({
array2: [Array2Schema]
})
Array2Schema = new mongoose.Schema({
answeredBy": [...]
})
This way the object will look like the one you show, but now each array are filled with sub-documents. This makes it possible to dot your way into the sub-document you want. Instead of using a .update you then use a .find or .findOne to get the document you want to update.
Main.findOne((
{
_id: 1
}
)
.exec(
function(err, result){
result.array1.id(12).array2.id(123).answeredBy.push('success')
result.save(function(err){
console.log(result)
});
}
)
Haven't used the .push() function this way myself, so the syntax might not be right, but I have used both .set() and .remove(), and both works perfectly fine.