Let's say I have the following data model in Mongo:
{
_id: ...,
name: "...",
obj: {...},
list: [ ... ],
}
Now let's say, my list array is very long, and I don't want to grab the whole document every time. So I want to get obj and name, but only grab the last 5 elements in list. How do you do this with with Mongo? I'm using pymongo.
I think you are looking for the $slice operator. Docs are here.
The syntax you are looking for is something like this:
db.coll.find({}, {obj:1, name: 1, list:{$slice: -5}}); // last 5
Note that this will also return the _id field by default. If you do not want the _id add _id:0 in front of obj:1. This is the JS syntax, but the python syntax will be very close.
use $slice operator to limit array elements
GeoLocation.find({},{name: 1, geolocation:{$slice: -5}})
.then((result) => {
res.json(result);
})
.catch((err) => {
res.status(500).json({ success: false, msg: `Something went wrong. ${err}` });
});
where geolocation is array of data, from that we get last 5 record.
Related
I'm in a trouble. I want to update the second item in a sorted doc in mongo. But the UpdateOne can't use skip in the filter param. I know the aggreate operation cant do it with skip and merge. However, how to use UpdateOne to realize it?
the data:
[
{"_id":1, "name": "tramp"},
{"_id":2, "name": "Biden"},
...
]
How to update them _id equal to 2, 3, and others?
thanks.
Even so, I'm not entirely clear about your query. I hope you'll find what you're looking for here.
const { data } = req.body;
data.forEach(async (e) => {
await User.findByIdAndUpdate({ _id: e._id }, { name: e.name});
})
User is your Collection Name
I have an Collection Model that has a property of items that holds an array of Item Models.
const CollectionSchema = new mongoose.Schema({
title: {
type: String,
required: true
},
items : [{type : mongoose.Schema.Types.ObjectId, ref: 'Item'}]
});
I tried to populate the items array in order to get the objectId's properties, but the items array would return back empty. (The code below is how I populated the items array. I first found the collection I was looking for by the _id using the req.body.search. I then ran .populate("items") in order to populate the items array. What I got back was an empty items array.)
userRouter.post('/iList', passport.authenticate('jwt', {session: false}), (req, res) => {
Collection.findById({_id : req.body.search}).populate("items").exec((err, document) => {
if(err)
res.json(err)
else
res.json({item: document})
})
});
I know my items array isn't empty since I can check on mongoDB that it is full.
The image of my mongoDB collection with an items array that isn't empty.
The weird thing is that if I put "collections" into the .populate params, my Items array does return with stuff, but it only returns the ObjectIDs and not actual object properties. I am confused to why .populate("items") isn't working.
If you are using findById then why are you specifying {_id: req.body.search}. If your req.body.search is a type of mongoose ObjectId string then you can directly use findById(req.body.search) instead of that. Also you can use simply the projection. Second argument in find calls are projections
If you are trying get items array only then why don't you try this query:-
Collection.findById(req.body.search, {items: 1}).then((result) => {
console.log('Items are :-\n', result);
}, (err) => {
console.log(err);
})
1 means include and 0 means exclude. So items will be present in output, also _id is default in output. In case you want to exclude _id then you can change second parameter to this -> {items: 1, _id: 0}
Never mind. The issue was when I pushed the Item Models via mongoose, I forgot to do items.save() which meant the items array held nothing.
I'm trying to update an array that sits inside another array in a document. The schema is like this:
const projectSchema = new mongoose.Schema({
stakeholders: [{
stakeholderTitle: {
type: String,
},
...
subgroup: [{
subgroupTitle: {
type: String
},
subgroupPercent: {
type: Number,
}
}]
}],
and I'm trying to update the 'subgroup' array. I have got the query to work on its parent (the stakeholder array) with the positional $ operator, using the answer to this question I asked previously. So my query looks like this.....
await db.findOneAndUpdate({ find by the id }, { "stakeholders.$.stakeholderTitle": req.body.stakeholderTitle, ... "stakeholders.$.subgroup": req.body.subgroup })
However, this query doesn't work for the 'stakeholders subgroup' array, and makes it null. Looking through the mongo docs for the positional operator it states that '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', which I guess might be my problem.
So how can I do this with a findOneAndUpdate query?
From what I see is you have to specify the object you want to update inside the subgroup array. Try this - (i.e I'm updating the subgroupTitle of the subgroup array);
await db.findOneAndUpdate(
{
_id: userId,
"stakeholders.stakeholderTitle": req.body.stakeholderTitle,
"stakeholders.stakeholderTitle.subgroup.subgroupTitle": req.body.subgroupTitle
},
{$set: {
"stakeholders.stakeholderTitle.subgroup.$.subgroupPercent": somePercentValue,
}
},
);
Also note, it's only the array that you find that you can update. It might not be exactly what you want, but its a step closer
I wish to return just the document id's from mongo that match a find() query.
I know I can pass an object to exclude or include in the result set, however I cannot find a way to just return the _id field.
My thought process is returning just this bit of information is going to be way more efficient (my use case requires no other document data just the ObjectId).
An example query that I expected to work was:
collection.find({}, { _id: 1 }).toArray(function(err, docs) {
...
}
However this returns the entire document and not just the _id field.
You just need to use a projection to find what ya want.
collection.find({filter criteria here}, {foo: 0, bar: 0, _id: 1});
Since I don't know what your document collection looks like this is all I can do for you. foo: 0 for example is exclude this property.
I found that using the cursor object directly I can specify the required projection. The mongodb package on npm when calling toArray() is returning the entire document regardless of the projection specified in the initial find(). Fixed working example below that satisfies my requirements of just getting the _id field.
Example document:
{
_id: new ObjectId(...),
test1: "hello",
test2: "world!"
}
Working Projection
var cursor = collection.find({});
cursor.project({
test1: 0,
test2: 0
});
cursor.toArray(function(err, docs) {
// Importantly the docs objects here only
// have the field _id
});
Because _id is by definition unique, you can use distinct to get an array of the _id values of all documents as:
collection.distinct('_id', function(err, ids) {
...
}
you can do like this
collection.find({},'_id').toArray(function(err, docs) {
...
}
How can I get an array of all the doc ids in MongoDB? I only need a set of ids but not the doc contents.
You can do this in the Mongo shell by calling map on the cursor like this:
var a = db.c.find({}, {_id:1}).map(function(item){ return item._id; })
The result is that a is an array of just the _id values.
The way it works in Node is similar.
(This is MongoDB Node driver v2.2, and Node v6.7.0)
db.collection('...')
.find(...)
.project( {_id: 1} )
.map(x => x._id)
.toArray();
Remember to put map before toArray as this map is NOT the JavaScript map function, but it is the one provided by MongoDB and it runs within the database before the cursor is returned.
One way is to simply use the runCommand API.
db.runCommand ( { distinct: "distinct", key: "_id" } )
which gives you something like this:
{
"values" : [
ObjectId("54cfcf93e2b8994c25077924"),
ObjectId("54d672d819f899c704b21ef4"),
ObjectId("54d6732319f899c704b21ef5"),
ObjectId("54d6732319f899c704b21ef6"),
ObjectId("54d6732319f899c704b21ef7"),
ObjectId("54d6732319f899c704b21ef8"),
ObjectId("54d6732319f899c704b21ef9")
],
"stats" : {
"n" : 7,
"nscanned" : 7,
"nscannedObjects" : 0,
"timems" : 2,
"cursor" : "DistinctCursor"
},
"ok" : 1
}
However, there's an even nicer way using the actual distinct API:
var ids = db.distinct.distinct('_id', {}, {});
which just gives you an array of ids:
[
ObjectId("54cfcf93e2b8994c25077924"),
ObjectId("54d672d819f899c704b21ef4"),
ObjectId("54d6732319f899c704b21ef5"),
ObjectId("54d6732319f899c704b21ef6"),
ObjectId("54d6732319f899c704b21ef7"),
ObjectId("54d6732319f899c704b21ef8"),
ObjectId("54d6732319f899c704b21ef9")
]
Not sure about the first version, but the latter is definitely supported in the Node.js driver (which I saw you mention you wanted to use). That would look something like this:
db.collection('c').distinct('_id', {}, {}, function (err, result) {
// result is your array of ids
})
I also was wondering how to do this with the MongoDB Node.JS driver, like #user2793120. Someone else said he should iterate through the results with .each which seemed highly inefficient to me. I used MongoDB's aggregation instead:
myCollection.aggregate([
{$match: {ANY SEARCHING CRITERIA FOLLOWING $match'S RULES} },
{$sort: {ANY SORTING CRITERIA, FOLLOWING $sort'S RULES}},
{$group: {_id:null, ids: {$addToSet: "$_id"}}}
]).exec()
The sorting phase is optional. The match one as well if you want all the collection's _ids. If you console.log the result, you'd see something like:
[ { _id: null, ids: [ '56e05a832f3caaf218b57a90', '56e05a832f3caaf218b57a91', '56e05a832f3caaf218b57a92' ] } ]
Then just use the contents of result[0].ids somewhere else.
The key part here is the $group section. You must define a value of null for _id (otherwise, the aggregation will crash), and create a new array field with all the _ids. If you don't mind having duplicated ids (according to your search criteria used in the $match phase, and assuming you are grouping a field other than _id which also has another document _id), you can use $push instead of $addToSet.
Another way to do this on mongo console could be:
var arr=[]
db.c.find({},{_id:1}).forEach(function(doc){arr.push(doc._id)})
printjson(arr)
Hope that helps!!!
Thanks!!!
I struggled with this for a long time, and I'm answering this because I've got an important hint. It seemed obvious that:
db.c.find({},{_id:1});
would be the answer.
It worked, sort of. It would find the first 101 documents and then the application would pause. I didn't let it keep going. This was both in Java using MongoOperations and also on the Mongo command line.
I looked at the mongo logs and saw it's doing a colscan, on a big collection of big documents. I thought, crazy, I'm projecting the _id which is always indexed so why would it attempt a colscan?
I have no idea why it would do that, but the solution is simple:
db.c.find({},{_id:1}).hint({_id:1});
or in Java:
query.withHint("{_id:1}");
Then it was able to proceed along as normal, using stream style:
createStreamFromIterator(mongoOperations.stream(query, MortgageDocument.class)).
map(MortgageDocument::getId).forEach(transformer);
Mongo can do some good things and it can also get stuck in really confusing ways. At least that's my experience so far.
Try with an agregation pipeline, like this:
db.collection.aggregate([
{ $match: { deletedAt: null }},
{ $group: { _id: "$_id"}}
])
this gona return a documents array with this structure
_id: ObjectId("5fc98977fda32e3458c97edd")
i had a similar requirement to get ids for a collection with 50+ million rows. I tried many ways. Fastest way to get the ids turned out to be to do mongoexport with just the ids.
One of the above examples worked for me, with a minor tweak. I left out the second object, as I tried using with my Mongoose schema.
const idArray = await Model.distinct('_id', {}, function (err, result) {
// result is your array of ids
return result;
});