Avoiding duplicate inserts?

I'm wondering if there is a good Rails solution to avoiding duplicate
inserts with identical content. I know of the locking to prevent
updates, but looking for something on inserts.

For example, say you have an API to a blog system that lets a caller
post a comment. The client calls "addComment" with the same parameters
(say, user_id and comment) twice and each request is handled by a
different mongrel instance. How can the duplicate comment be prevented
from being saved? They would be different rows, and I would like to
avoid table locking. (In this example, the client can't be controlled,
so it needs to be server-side.)

I believe that a validating on before_save is not enough because there
would still be a window between the validation completing and writing
the row since it is only the row that is locked.

Any suggestions would be appreciated.

Thanks,

-mike

not sure if that would work in your case, but what about unique keys in
your database?

Sounds like a job for validates_uniqueness_of :comment, :scope=>:user

The double-submit issue is a matter to solve on the client (e.g.,
disable the submit button after it's clicked; clear the comment field,
etc).

For now, I'd recommend that you take this simple approach and let the
real users' use demonstrate that more needs to be done. I suspect
you're concern (beyond this) is more theoretical than practical.