how to get the max of my collection without the _id

[MANU]

Hi everybody,

I developped a map/reduce with the aggregation framework of mongo.

Here is my data :

db.coll.save({a:'Martin',b:'wesh',c:5})
db.coll.save({a:'Martin',b:'wesh',c:15})
db.coll.save({a:'Martin',b:'wesh',c:2})
db.coll.save({a:'Manu',b:'bri',c:1})
db.coll.save({a:'Manu',b:'bri',c:2})
db.coll.save({a:'Manu',b:'bri',c:10})
db.coll.save({a:'Manu',b:'bri',c:6})

The result I want :

martin wesh : 2
Manu bri : 1

Here is my code :

db.coll.group(
{key: { a:true, b:true },
reduce: function(obj,prev) { if (prev.min > obj.c)
{prev.min = obj.c}; },
initial: { min: 15 }
});

I want to replace : " initial: { min: 15 } " BY " initial: { min:
db.coll.find({},{c:1}).sort({c:-1}).limit(1) } "

but the last way return {"_id" : 5654653 , "c" : 15} and not only 15

What's the solution guys ?

Thanks a lot.

Best regards

[Sam Millman]

I don't it's possible and nor would it be reliable since the sorting would be undone in the grouping itself.

Best thing is to use a sorting algorithm on the returned BSON object to sort your group results by your desired field.

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[Dan Crosta]

It looks like you're actually computing the minimum "c" value for each person, not the maximum -- was that just a typo in your subject line?

As for the particular question, you can use javascript's "typeof" operator to ensure that the first value for any given person is taken as the minimum, and then that any subsequent value is taken if it is smaller than the current value. By the time the group() call finishes, this will give you the minimum for each person:

db.coll.group({

    key: {a: true, b: true},

    reduce: function (obj, prev) {

        if (obj.c < prev.min || typeof(prev.min) == "undefined") {

            prev.min = obj.c;

        }

    },

    initial: {}

})

Hope this helps,

- Dan

[Sam Millman]

Wait I think you may have been asking somehting else now that I look at:

"but the last way return {"_id" : 5654653 , "c" : 15} and not only 15"

Do you mean it return _id in the group() function from doing that? It appears I completely misread.

You might be able to solve this with adding _id:0 but it will still output {c:15} as such you may want to use Dan's approach.

--

[MANU]

the Dan's solution is perfect.

Thank you guys.

A last things about my probleme :
isn't it possible to do something like that ? :

var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1) ;

print a[1]

In order to print the value 15 only ???

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[Sam Millman]

No but you can do: a[1][c]

mongodb-user...@googlegroups.com

[MANU]

I've got this result :

> var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1)

> a
{ "_id" : ObjectId("4fbf9bf2ab98db2e62486a3a"), "c" : 15 }

> var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1)

> a[1][c]
Fri May 25 17:08:22 ReferenceError: c is not defined (shell):1

> var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1)

> a[1]["c"]
Fri May 25 17:08:58 TypeError: a[1] has no properties (shell):1

I'd like to do :

>a/3
5

[Sam Millman]

Ooooh, I see now. Yea the problem is that it's returning an array (or cursor, depending on where your doing that).

Hmmmm, a[0]["c"] is prolly the best way, if your dong it from console?

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[Sam Millman]

Sorry it seems a["c"] will do it according to your output since the console does that for you

[MANU]

Fine !
thanks a lot

Fri May 25 17:08:58 TypeError: a[1] has no properties (shell):1

Le vendredi 25 mai 2012 17:15:29 UTC+2, Sammaye a écrit :

Ooooh, I see now. Yea the problem is that it's returning an array (or cursor, depending on where your doing that).

Hmmmm, a[0]["c"] is prolly the best way, if your dong it from console?

On 25 May 2012 16:10, MANU  wrote:

I've got this result :

> var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1)

> a
{ "_id" : ObjectId("4fbf9bf2ab98db2e62486a3a"), "c" : 15 }

> var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1)

> a[1][c]
Fri May 25 17:08:22 ReferenceError: c is not defined (shell):1

> var a = db.coll.find({},{c:1}).sort({c:-1}).limit(1)

> a[1]["c"]
Fri May 25 17:08:58 TypeError: a[1] has no properties (shell):1

I'd like to do :

>a/3
5


Le vendredi 25 mai 2012 17:00:57 UTC+2, Sammaye a écrit :

No but you can do: a[1][c]

[Sam Millman]

Though do be aware that this behaviour does not transist into all drivers, so in PHP even with a limit one you will get a cursor back.

Just thought I'd let you know incase your banking on this functionality within your server side language.

[MANU]

for the moment I'm currently in the console.

My objective is to compare performance query compare to Oracle's query GROUP BY.

So my second step will be putting my mongo code into a js script and analyse the time spent.

Is it possible to write into a file in javascript in order to store the start date and the end date ?

[Dan Crosta]

You can store the date into a variable using something like:

var start = new Date()
/* do some stuff */
var end = new Date()
print("took " + ((end - start) / 1000) + " seconds");

If you are comparing performance, you may also want to compare against
map-reduce and the aggregation framework. This particular algorithm
(finding the max or min value within a group) can be implemented in
all 3 approaches.

- Dan

[MANU]

Thank you but I don't want to print my result into the log of my console.

I prefer write my results into a log file with some information like the methode, the number of document.

Can I do something like that :

var myFile = document.open()

var start = new Date()

myFile.write(start)

/* do some stuff */
var end = new Date()

myFile.write(end)

[Dan Crosta]

There's no particularly good way to do this from within the MongoDB
shell, but you can redirect the shell's printed output into a file.
For instance, if you are using a Linux-like system, you could do:

mongo host:port/dbname yourScript.js > scriptResults.txt

- Dan

[MANU]

Yes !
you made my day guys ! ;)

thanks for all and have a nice WK