I’m running into a weird issue with my Rails 3 application. I have an email forwarding service that sends POST requests to my app when new emails arrive. Everything worked fine in Rails 2, but after upgrading to Rails 3, I keep getting this error.
The error message shows undefined method 'rewind' for #<String:0x...> and it happens every time the service tries to post data to my controller. I think this might be a compatibility issue with how Rails 3 handles incoming request data compared to the previous version.
Has anyone else experienced similar problems when processing external HTTP POST requests in Rails 3? The same code worked perfectly before the upgrade, so I’m pretty sure it’s related to changes in the framework.
class MessagesController < ApplicationController
def process_email
# This is where the error occurs when external service posts
email_data = params[:message_content]
sender = params[:from_address]
# Process the email content
new_message = EmailMessage.create(
content: email_data,
sender_email: sender
)
render json: { status: 'received' }
end
end
The stack trace shows it’s failing during parameter processing, but I can’t figure out what’s causing the rewind method to be called on a String object.
Classic Rails 3 migration headache. Rails 3 changed how it parses request bodies, especially multipart data from external services.
Your email service is sending data that Rails 3 expects as a file-like object (needs rewind method), but you’re getting a regular string instead.
Don’t fight Rails parsing quirks - move this to a proper automation platform. I’ve dealt with similar email processing and found dedicated tools way more reliable than wrestling with Rails internals.
Set up a webhook that catches your email service posts, transforms the data, then sends clean API calls to your Rails app. No more parameter parsing nightmares, plus you get better data validation control.
Bonus: you can add retry logic, logging, and route emails to different handlers based on content or sender. Way cleaner than debugging Rails guts.
Sounds like middleware is messing with your email service posts. Check for any Rack middleware that’s parsing the request body before it reaches your controller - that’s usually what causes rewind errors in Rails 3. Try adding protect_from_forgery :except => :process_email and see if that helps. Also double-check that your email service isn’t sending funky encoding headers.
Been there with Rails 3 upgrades breaking external service integrations. The rewind issue happens when Rails treats string parameters as IO streams during multipart parsing. Your email service’s POST format makes Rails expect a rewindable object but gets plain strings instead.
First, debug the raw request in your controller to see what’s actually coming in. I had luck using request.raw_post directly instead of letting params parsing handle external service data. You can manually parse the POST body with Rack::Utils.parse_query(request.raw_post) to skip Rails automatic parsing.
Also check your routes - you might have CSRF protection or other filters messing with external requests. Adding skip_before_filter :verify_authenticity_token to your messages controller often fixes these integration issues.
Hit this exact problem during my Rails 3 upgrade last year. Rails 3 changed how it parses request bodies, especially multipart content from external services. The rewind error happens because your email service sends data that Rails thinks needs file-like IO operations, but it’s getting a plain string. Usually means the Content-Type headers don’t match what Rails expects. Quick fix that worked for me: add a before_filter to your controller that handles the raw request body before Rails tries parsing it. Try request.body.rewind if request.body.respond_to?(:rewind) in a rescue block around your parameter access. Or check what Content-Type your email service sends. If it’s multipart/form-data with string content, configure your service to use application/x-www-form-urlencoded instead. Most email forwarding services have this option in their webhook settings.