In MongoDB I have some documents such as these :
{type:'a', ts:123, foo:'bar'}
{type:'a', ts:456, foo:'baz'}
{type:'a', ts:789, foo:'whee'}
{type:'b', ts:123, foo:'jingle'}
{type:'b', ts:456, foo:'bells'}
{type:'b', ts:789, foo:'yeehaaw'}
I would like to find the documents which have the max ts value for each type.
Is it possible to do that with a disngle query ?
I've looked at distinct but apparently it only returns the disting 'type' values.
So I guess I would need a combination of $max alongside distinct ...
But I can't figure the proper way.
Thanks
Related
I have a mongodb query where i want to get documents if a field has particular value.
db.collection.find({key:{$in:['value1','value2']}}) if i run above command i get documents containing either 'value1' or 'value2'. but lets just say there are no values. and i search db.collection.find({key:{$in:[]}}), nothing is displayed. and db.collection.find({key:{$in:[*]}}) gives unexpected token* which wild card do i use in $in to show all results.?
I think this is logically consistent behavior for $in. The query
db.collection.find({ "key" : { "$in" : [] } })
could be translated as "find all the documents where the value of key is one of the values contained in the array []". Since there are no values in the array [], there are no matching documents. If you want to find all of the extant values for key, use .distinct to return them as an array:
db.collection.distinct("key")
.distinct will use an index if possible.
If you want a query to match all documents, omit the query selector from .find:
db.collection.find()
as suggested in the comments.
I have a mongo DB collection that looks something like this:
{
{
_id: objectId('aabbccddeeff'),
objectName: 'MyFirstObject',
objectLength: 0xDEADBEEF,
objectSource: 'Source1',
accessCounter: {
'firstLocationCode' : 283,
'secondLocationCode' : 543,
'ThirdLocationCode' : 564,
'FourthLocationCode' : 12,
}
}
...
}
Now, assuming that this is not the only record in the collection and that most/all of the documents contain the accessCounter subdocument/field how will I go with selecting the x first documents where I have the most access from a specific location.
A sample "query" will be something like:
"Select the first 10 documents From myCollection where the accessCounter.firstLocationCode are the highest"
So a sample result will be X documents where the accessCounter. will be the greatest is the database.
Thank your for taking the time to read my question.
No need for an aggregation, that is a basic query:
db.collection.find().sort({"accessCounter.firstLocation":-1}).limit(10)
In order to speed this up, you should create a subdocument index on accessCounter first:
db.collection.ensureIndex({'accessCounter':-1})
assuming the you want to do the same query for all locations. In case you only want to query firstLocation, create the index on accessCounter.firstLocation.
You can speed this up further in case you only need the accessCounter value by making this a so called covered query, a query of which the values to return come from the index itself. For example, when you have the subdocument indexed and you query for the top secondLocations, you should be able to do a covered query with:
db.collection.find({},{_id:0,"accessCounter.secondLocation":1})
.sort("accessCounter.secondLocation":-1).limit(10)
which translates to "Get all documents ('{}'), don't return the _id field as you do by default ('_id:0'), get only the 'accessCounter.secondLocation' field ('accessCounter.secondLocation:1'). Sort the returned values in descending order and give me the first ten."
I have a query where the criteria is as follow (PHP):
...
->findBy(['userId'=>new \MongoId($me),'active'=>1],array('date' => 'DESC'),10);
I'm wondering whether it would be more efficient to use a triple index than using a simple index on userId for example.
My other question is : the field active can change; does it still make sense to create an index on this field ? if yes , how do indexes update their values ?
You can use the mongodb profiler, to find which of your queries are slow & then index the fields that have a 'where' clause, example
db.system.profile.find( { millis : { $gt : 5 } } ).pretty()
http://docs.mongodb.org/manual/tutorial/manage-the-database-profiler/
I am using mongo's shell and want to do what is basically equivalent to "SQL's select col INTO var" and then use the value of var to look up other rows in the same table or others (Joins). For example, in PL/SQL I will declare a variable called V_Dno. I also have a table called Emp(EID, Name, Sal, Dno). I can access the value of Dno for employee 100 as, "Select Dno into V_Dno from Emp where EID = 100). In MongoDB, when I find the needed employee (using its _id), I end up with a document and not a value (a field). In a sense, I get equivalent to the entire row in SQL and not just a column. I am doing the following to find the given emp:
VAR V_Dno = db.emp.find ({Eid : 100}, {Dno : 1});
The reason I want to do this to traverse from one document into the other using the value of a field. I know I can do it using the DBRef, but I wanted to see if I could tie documents together using this method.
Can someone please shed some light on this?
Thanks.
find returns a cursor that lets you iterate over the matching documents. In this case you'd want to use findOne instead as it directly returns the first matching doc, and then use dot notation to access the single field.
var V_Dno = db.emp.findOne({Eid : 100}, {Dno : 1}).Dno;
Using your query as a starting point:
var vdno = db.emp.findOne({Eid: 100, Dno :1})
This returns a document from the emp collection where the Eid = 100 and the Dno = 1. Now that I have this document in the vdno variable I can "join" it to another collection. Lets say you have a Department collection, a document in the department collection has a manual reference to the _id field in the emp collection. You can use the following to filter results from the department collection based on the value in your variable.
db.department.find({"employee._id":vdno._id})
How to get maximum value for column in mongoDB using mongoTemplate object in spring
I am guessing you are not talking about query syntex. If you are talking about writing performant query, then you can choose to have index on that field.
ensureIndex({ coll-name : 1 })
Query
db.coll.find({}, {coll-name : 1, _id : 0}).sort({ coll-name : -1 }).limit(1)
This will return you a json having max-value for coll-name.
Abhishek's answer doesn't address Spring and how to do this in MongoTemplate.
Mongo will optimize sort/limit combinations IF the sort field is indexed (or the #Id field). Otherwise it is still pretty good because it will use a top-k algorithm and avoid the global sort (mongodb sort doc). This is from Mkyong's example but I do the sort first and set the limit to one second.
Query query = new Query();
query.with(new Sort(Sort.Direction.DESC, "idField"));
query.limit(1);
MyObject maxObject = mongoTemplate.findOne(query, MyObject.class);