I would like to count the number of documents returned by an aggregation.
I'm sure my initial aggregation works, because I use it later in my programm. To do so I created a pipeline variable (here called pipelineTest, ask me if you want to see it in detail, but it's quite long, that's why I don't give the lines here).
To count the number of documents returned, I push my pipeline with :
{$count: "totalCount"}
Now I would like to get (or log) totalCount value. What should I do ?
Here is the aggregation :
pipelineTest.push({$count: "totalCount"});
cursorTest = collection.aggregate(pipelineTest, options)
console.log(cursorTest.?)
Thanks for your help, I read lot and lot doc about aggregation and I still don't understand how to read the result of an aggregation...
Assuming you're using async/await syntax - you need to await on the result of the aggregation.
You can convert the cursor to an array, get the first element of that array and access totalCount.
pipelineTest.push({$count: "totalCount"});
cursorTest = await collection.aggregate(pipelineTest, options).toArray();
console.log(cursorTest[0].totalCount);
Aggregation
db.mycollection.aggregate([
{
$count: "totalCount"
}
])
Result
[ { totalCount: 3 } ]
Your Particulars
Try the following:
pipelineTest.push({$count: "totalCount"});
cursorTest = collection.aggregate(pipelineTest, options)
console.log(cursorTest.totalCount)
I have been trying for a while to extract the insertion date of a mongodb document and add it as a new field to the same document.
I'm trying to do it using the mongo and mongo shell aggregation framework without getting good results.
Here is my query
db.getCollection('my_collection').aggregate(
[
{
$match: {
MY QUERY CRITERIA
}
},
{
$addFields: { "insertTime": "$_id.getTimestamp()" }
}
]
)
I am trying to extracr insertion time from _id using the function getTimestamp() but for sure there is somtehing about aggregation framework syntax that I am missing because I can not do what I am trying to do in my query.
This works perfect:
ObjectId("5c34f746ccb26800019edd53").getTimestamp()
ISODate("2019-01-08T19:17:26Z")
But this does not work at all:
"$_id.getTimestamp()"
What I am missing?
Thanks in advance
Is there a way to push all the documents of a given collection in a array?
I did this but is there any quicker way?
var ops = [];
db.getCollection('stock').find({}).forEach(function (stock) {
ops.push(stock);
})
PS: I use Mongo 3.4
You can just use the toArray function on the cursor that's returned from find, like this:
var ops = db.getCollection('stock').find({}).toArray();
Note: As with your original solution, this might suffer with performance if the stock collection contains millions of documents.
As an aside, you can use db.stock directly to shorten the query a little bit:
var ops = db.stock.find({}).toArray();
Try using lean query option. in your case:
db.getCollection('stock').find({}).lean()
You could as well use $facet which will allow you to create the array on the server side - provided the resulting document array is no bigger than 16MB in which case you'll get an exception:
db.stock.aggregate({
$facet: {
ops: [ { $match: {} } ]
}
})
In order to reduce the amount of data returned you could limit the number of returned fields in the above pipeline (instead of an empty $match stage - which is a hack anyway - you would then use $project).
I have produced some results through mongodb aggregation framework. So now I need to get those results to a file(text or any other format).
How can I achieve this ?
Check the $out operator in aggregation framework.
$out operator takes the documents returned by the aggregation pipeline and writes
them to a specified collection. The $out operator lets the aggregation
framework return result sets of any size. The $out operator must be
the last stage in the pipeline.
The command has the following syntax, where is
collection that will hold the output of the aggregation operation.
$out is only permissible at the end of the pipeline:
db.<collection>.aggregate( [
{ <operation> },
{ <operation> },
...,
{ $out : "<output-collection>" }
] )
Check my answer to a previous question how to export output of the aggregation framework to a new collection. After exporting to a new collection you can simply do mongodump.
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;
});