a way to check if a document exists in a collection?

[oferfort]

Hey,
I'm trying to figure out the best way (regarding performance) to check
if a document with a certain _id exists in a collection.
I know i can just query for it, but that doesn't seem too efficient.
I guess my question is this:
if i do db.collection.find({_id: 12345}, {_id: 1}), will it not go to
the disk at all, since the _id is in memory already, and just return
12345 if the document exists, or null if it doesn't?

thanks

[Scott Hernandez]

That is probably a good query but I don't think the server will return
data from the index for the document. But when it does that will be
the most efficient. You can follow
http://jira.mongodb.org/browse/SERVER-192

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[oferfort]

thanks,
let me continue with another question.

I'm trying to move some of my old data from my old MySQL to the mongo
server.
Most of the data was already moved, but some of it is not.
I'm trying to figure out what would be "cheaper":

1. Doing an insert to a document with the requested _id, and failing
internally if the _id already exist. (if it exists i don't want to
update it)
2. To query for the object, and to do an insert only if it doesn't
exist.

thanks

On Oct 25, 2:19 am, Scott Hernandez <scotthernan...@gmail.com> wrote:
> That is probably a good query but I don't think the server will return
> data from the index for the document. But when it does that will be

> the most efficient. You can followhttp://jira.mongodb.org/browse/SERVER-192

[Eliot Horowitz]

Depends on the % that's already there. If its mostly there already,
querying will be faster. If its mostly not there, then just inserting
will be faster.

[Ofer Fort]

most of it is already there, so i guess i'll go with the query
thanks