How to use for loop in mongodb

Question

I need to insert a new field(column) to mongodb collection which has now 5246 documents. The field should be auto incremented . So that i use an for loop . My query is as follows `

for(i=1;i<=5246;i++) {
    db.coll.update({},{$set:{"new_field":i}},false,true)
};

But my bad the output is as,

{new_field:5246},{new_field:5246},{new_field:5246},.......

Is there any problem with query..?.

You sure this is Java? Doesn't look like it.

Nick H
– Nick H

2012-10-18 08:39:41 +00:00
Commented Oct 18, 2012 at 8:39 — Nick H
– Nick H, Commented Oct 18, 2012 at 8:39

Sammaye · Accepted Answer · 2012-10-18 09:28:46Z

20

Why are you updating all records with no find criteria? Technically this loop is working as it should. What you need to do instead is loop through a cursor of your collection like so:

var cursor = db.coll.find(),
    i = 0;

cursor.forEach(function(x){
    db.coll.update({_id: x._id}, {$set:{new_field:i}})
    $i++;
});

Something like that would work instead.

edited Oct 18, 2012 at 9:28

answered Oct 18, 2012 at 9:02

Sammaye

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7 Comments

Mohammed shebin Over a year ago

Thnks fr the respnse. But what this 'row in cursor' stands for.?

Sammaye Over a year ago

@Mohammedshebin Each cursor is an object which when used like an array, in a foreach loop in most languages, will output a row which represents a row in your database. It is equal to using forEach on the rows. Infact you could replace the for for a forEach here.

Mohammed shebin Over a year ago

ok. will u pls send me the exact code ..? also what u mnt by row._id ..?

Sammaye Over a year ago

@Mohammedshebin This is a simple loop, don't you know how to do loops? row._id is accessing the _id of that row you pull out. I have changed my code to work from the forEach function instead, it should work out the box.

Sammaye Over a year ago

@Mohammedshebin You cna also use the next functions as displayed here: mongodb.org/display/DOCS/Queries+and+Cursors

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