Ruby-on-rails – How to turn off rails protect_from_forgery filter only for json

I have web site built with Rails3 and now I want to implement json API for mobile client access. However, sending json post request from the client because of the protect_from_forgery filter. Because the client will not retrieve any data from the server, there is no way that the client can receive auth_token so I would like to turn off the protect_from_forgery option only for json requests (I thought rails3 does this in default but apparently it does not).

I know similar topic is discussed at here but in that case, he receives auth_token before sending post request.

So my question is turning off the protect_from_forgery only for json is good way of doing this? If yes, how to do so? If no, what is the alternative?

FYI, I use following ajax request

$.ajax({
             type: 'POST',  
             url: "http://www.example.com/login.json",  
             data: { 'email': emailVal, 'password': passwordVal },  
             success: onSuccess,  
             error: onError,  
             dataType: "json"  
});

and I get ActionController::InvalidAuthenticityToken error.

By the way, following curl command works though…

curl -H "Content-Type: application/json" -c cookies.txt -d '{"email": emailVal, "password": passwordVal}' -X POST http://www.example.com/login.json -i