query on the length of a string

[Steve]

Hi,

How do i write a query that returns the count of docs that have a
string field less than 74 characters?

e.g., something like (where foobar is a string field):

db.my_col.count( { foobar : {$length : {$lt : 74}} })

Thanks!

[Robert Stam]

You could use a $where query. For example:

> db.test.remove()
> db.test.insert({s:"1234"})
> db.test.insert({s:"12345678"})
> db.test.find({$where:"this.s.length < 5"})
{ "_id" : ObjectId("4d5ed1fb90dd484ca8a33190"), "s" : "1234" }

[Scott Hernandez]

If you want this to be faster you can store the length of the string
as another field and then you can index or search on it; $where is
going to be pretty slow compared to that.

Generally javascript and $where are a last resort when you can't
structure the data in any other way, or when you are dealing with a
small subset of data.

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[Steve]

thanks Robert,

here is the query i ended up using:

db.test.count( {$where:"this.s.length < 74", created_at: {$gte: new
Date(2011,0,1), $lt: new Date(2011,1,1)}} )

this is pretty cool!! looking at the execution plan, it used the b-
tree index (we have on created_at) properly.

[Shi Shei]

Kevin, in your case the follwing query would be more performant:
db.Story.count({body:{$type:2}, $where:"this.body.length > 40000"});
or:
db.Story.count({body:{$exists:true}, $where:"this.body.length >
40000"});

This is because MongoDB will evaluate non-$where components of the
query before $where statements and non-$where query statements may use
an index.

On Jan 16, 9:09 pm, Kevin Lieberman <ke...@politear.com> wrote:
> In my case where the field also might not exist I had to check for the
> field's existence and then check the length like:
> db.Story.count({$where:"this.body && this.body.length > 40000"});