Since every collection in mongodb has a default index on the _id column, I wanted to leverage it for my scenario as below.
I have my collection as below,
{
"_id":{
"timestamp" : ISODate("2016-08-24T23:22:20.201Z"),
"departmentname" : "sales",
"city":"NJ"
}
//Other fields in my collection
}
With this structure I am able to query as below,
db.getCollection('test').find(
{
"_id" : {
"timestamp" : ISODate("2016-08-21T23:22:20.201Z"),
"departmentname" : "sales",
"city":"NJ"
}
}
)
But, when I query by one or more fields in which are part of _id column as below,
db.getCollection('test').find(
{
"_id" : {
"timestamp" : ISODate("2016-08-21T23:22:20.201Z")
}
}
)
(OR)
db.getCollection('test').find(
{
"_id" : {
"departmentname" : "sales"
}
}
)
(OR)
db.getCollection('test').find(
{
"_id" : {
"departmentname" : "sales",
"city":"NJ"
}
}
)
I do not see any documents returned
When I checked with .explain() I see that it has used Index but did not find any documents.
Also, I would like to do date range queries on timestamp field along with query on one or more fields in the _id column like below,
db.getCollection('test').find(
{
"_id.timestamp" : {
"$gte": ISODate("2011-08-21T23:22:20.201Z")
},
"_id.departmentname" : "sales"
}
)
But, I do not see any documents returned. When I run .explain() I see it has used colscan and not index.
Can someone help me on the right way to query by one or more fields on my _id column.
Thanks,
Sri
You can try following query, in first case:-
db.getCollection('test').find(
{
"_id.timestamp" : ISODate("2016-08-21T23:22:20.201Z")
})
And this for multiple fields:
db.getCollection('test').find(
{
"_id.timestamp" : ISODate("2016-08-21T23:22:20.201Z"),
"_id.departmentname" : "sales",
})
Related
I have following students collection
{ "_id" : ObjectId("5f282eb2c5891296d8824130"), "name" : "Rajib", "mark" : "1000" }
{ "_id" : ObjectId("5f282eb2c5891296d8824131"), "name" : "Rahul", "mark" : "1200" }
{ "_id" : ObjectId("5f282eb2c5891296d8824132"), "name" : "Manoj", "mark" : "1000" }
{ "_id" : ObjectId("5f282eb2c5891296d8824133"), "name" : "Saroj", "mark" : "1400" }
My requirement is to sort the collection basing on 'mark' field in descending order. But it should not display 'mark' field in final result. Result should come as:
{ "name" : "Saroj" }
{ "name" : "Rahul" }
{ "name" : "Rajib" }
{ "name" : "Manoj" }
Following query I tried and it works fine.
db.students.find({},{"_id":0,"name":1}).sort({"mark":-1})
My MongoDB version is v4.2.8. Now question is what is the equivalent Aggregation Query of the above query. I tried following two queries. But both didn't give me desired result.
db.students.aggregate([{"$project":{"name":1,"_id":0}},{"$sort":{"mark":-1}}])
db.students.aggregate([{"$project":{"name":1,"_id":0,"mark":1}},{"$sort":{"mark":-1}}])
Why it is working in find()?
As per Cursor.Sort, When a set of results are both sorted and projected, the MongoDB query engine will always apply the sorting first.
Why it isn't working in aggregate()?
As per Aggregation Pipeline, The MongoDB aggregation pipeline consists of stages. Each stage transforms the documents as they pass through the pipeline. Pipeline stages do not need to produce one output document for every input document; e.g., some stages may generate new documents or filter out documents.
You need to correct:
You should change pipeline order, because if you have not selected mark field in $project then it will no longer available in further pipelines and it will not affect $sort operation.
db.students.aggregate([
{ "$sort": { "mark": -1 } },
{ "$project": { "name": 1, "_id": 0 } }
])
Playground: https://mongoplayground.net/p/xtgGl8AReeH
I did this in my mongodb:
db.teams.insert({name:"Alpha team",employees:[{name:"john"},{name:"david"}]});
db.teams.insert({name:"True team",employees:[{name:"oliver"},{name:"sam"}]});
db.teams.insert({name:"Blue team",employees:[{name:"jane"},{name:"raji"}]});
db.teams.find({"employees.name":/.*o.*/});
But what I got was:
{ "_id" : ObjectId("5ddf3ca83c182cc5354a15dd"), "name" : "Alpha team", "employees" : [ { "name" : "john" }, { "name" : "david" } ] }
{ "_id" : ObjectId("5ddf3ca93c182cc5354a15de"), "name" : "True team", "employees" : [ { "name" : "oliver" }, { "name" : "sam" } ] }
But what I really want is
[{"name":"john"},{"name":"oliver"}]
I'm having a hard time finding examples of this without using some kind of programmatic iterator/loop. Or examples I find return the parent document, which means I'd have to parse out the embedded array employees and do some kind of UNION statement?
Eg.
How to get embedded document in mongodb?
Retrieve only the queried element in an object array in MongoDB collection
Can someone point me in the right direction?
Please add projections to filter out the fields you don't need. Please refer the project link mongodb projections
Your find query should be constructed with the projection parameters like below:
db.teams.find({"employees.name":/.*o.*/}, {_id:0, "employees.name": 1});
This will return you:
[{"name":"john"},{"name":"oliver"}]
Can be solved with a simple aggregation pipeline.
db.teams.aggregate([
{$unwind : "$employees"},
{$match : {"employees.name":/.*o.*/}},
])
EDIT:
OP Wants to skip the parent fields. Modified query:
db.teams.aggregate([
{$unwind : "$employees"},
{$match : {"employees.name":/.*o.*/}},
{$project : {"name":"$employees.name",_id:0}}
])
Output:
{ "name" : "john" }
{ "name" : "oliver" }
I have queried mongodb by using aggregate function and got two fields in the output.
The result of my db.Collection.aggregate(..) looks like the below:
{
"_id" : NumberLong(203440),
"date" : ISODate("2013-05-11T00:00:00Z")
}
{
"_id" : NumberLong(203520),
"date" :ISODate("2013-01-05T00:00:00Z")
}
{
"_id" : NumberLong(203970),
"date": ISODate("2013-01-11T00:00:00Z")
}
{
"_id" : NumberLong(203660),
"date" : ISODate("2013-01-11T00:00:00Z")
}
{
"_id" : NumberLong(203360),
"date" : ISODate("2013-01-11T00:00:00Z")
}
How do I get the records in the collection for which these two fields are true?(in a single query)
i.e If each record in my collection has the fields data,_id, x, y, z, a , b and c,
how do I fetch list of records for which the date and _id are equal to the above result of aggregate?
In the aggregation command, using the below in the $group part helped me.
"allFields": {
"$first": "$$CURRENT"
}
In my response, I got the entire document in "allFields" field.
Let's say i want to return the latest inserted document from the subdocument. I want to be able to return the second record within the tags array w/ the _id of 54a1845def7572cd0e3fe288
So I far I have this query but it returns all values in the tags array.
db.modules.findOne({_id:"ui","svn_branches.branch":"Rocky"},{"svn_branches.$":1})
Mongodb array:
{
"_id" : "ui",
"svn_branches" : [
{
"updated_at" : ISODate("2013-06-12T20:48:17.297Z"),
"branch" : "Rocky",
"revision" : 0,
"tags" : [
{
"_id" : ObjectId("54a178b8ef7572d30e3fe288"),
"commit_message" : "r277 | ssmith | 2015-02-11 17:43:23 -0400 (Wed, 11 Feb 2015)",
"latest_tag" : "20150218r1_6.32_abc",
"revision" : 1,
"tag_revision_number" : "280",
"updated_at" : ISODate("2015-02-18T19:54:54.062Z")
},
{
"_id" : ObjectId("54a1845def7572cd0e3fe288"),
"commit_message" : "r271 | sam | 2dskjh\n",
"latest_tag" : "20150218r2_6.32_abc",
"revision" : 2,
"tag_revision_number" : "281",
"updated_at" : ISODate("2015-02-19T19:54:54.062Z")
}
]
}
]
}
Simple Solution
Let say we have a category as a document and items as a subdocument.
// find document from collection
const category = await Category.findOne({ _id:'$hec453d235xhHe4Y' });
// fetch last index of sub-document
const lastItemIndex = category.items.length - 1;
// here is the last item of sub-document
console.log(category.items[lastItemIndex]);
as mongodb inserted the latest sub-document at last index, so we need to find the last index for the latest sub-doc.
Queries in MongoDB do not return subdocuments (or, as in your case, subdocuments of subdocuments). They match and return the the documents in the collection. The documents' shape can be changed a bit by projection, but it's limited. If you want to find the latest tag commonly, you probably want to make your documents represent tags. Having an array in an array is generally a bad idea in MongoDB, too.
If this is an uncommon operation, and one that doesn't need to be particularly fast, you can use an aggregation:
db.modules.aggregate([
{ "$unwind" : "$svn_branches" },
{ "$unwind" : "$svn_branches.tags" },
{ "$sort" : { "svn_branches.tags.updated_at" : -1 } },
{ "$group" : { "_id" : "$_id", "latest_tag" : { "$first" : "$svn_branches.tags" } } }
])
I needed to find the last entry of subdocuments and I managed to make it to work with the $slice projection operator: mondodb.com > $slice (projection)
db.modules.find({_id:'ui', 'svn_branches.branch':'Rocky'},
{ 'svn_branches.tags': {$slice:-1} } )
I had only one level, if this doesn't work, please let me know.
> db.checklistDB.find({},{title:1, _id:0})
{ "title" : "Untitled Checklist" }
{ }
{ }
{ }
{ }
{ }
> db.checklistDB.find({},{title:1})
{ "title" : "Untitled Checklist", "_id" : "4FaJcAkzAY3Geyggm" }
{ "_id" : "3imNYy8SPcRDjLcqz" }
{ "_id" : "977fPtvEn7hiStqzp" }
{ "_id" : "QcAEMnr6R7qfaWFR8" }
{ "_id" : "eEsmKMdQGYKqnhTNB" }
{ "_id" : "cL6R8qxwWhvTr2kmy" }
Hi Guys,
As you can see from the above, I rand 2 commands:
db.checklistDB.find( {} , { title : 1 } ) and
*db.checklistDB.find( {} , { title : 1 , _id : 0} )
Both queries returns 6 records which is all the records that exists in the database. I would imagine that it will only return records that have "title" as a field. Is there something I'm doing wrong?
Second argument for find is projection. In your case it looks for title field inside document, and if it doesn’t exist returns none. If you want to filter documents you should use query like this:
db.checklistDB.find({title: {$exists: true}}, {title:1, _id:0})
EDIT
If you take your query:
db.checklistDB.find({}, {title:1, _id:0})
It translates to: retrieve all documents ({} as query argument) and for every document give me title if exist or default value (none) if it doesn’t ({title:1, _id:0} as projection argument). Projection argument is used only to transform not to filter documents.