Category: Technical Page 1 of 14

So maybe you have a model that is not backed by a database table? ActiveModel is meant to cover this scenario and give you the usual ActiveRecord goodness and validation.

But the story gets much harder if you want to have nested objects. In a normal ActiveRecord::Base backed model, you can simply say:

accepts_nested_attributes_for :order_lines

and Rails will manage form submitted nested parameters for you.

Life is not so simple, or documented with nested objects on ActiveModel. accepts_nested_attributes_for is not available. But some of the underpinnings are.

So enough talk, how do you make it work? I’ll show you with an Order / OrderLines example.

Note the very special name: order_lines_attributes=. This hooks into the Rails handling of nested form parameters. Also the valid? method propagates the child errors up to the parent object, so that they show up at the top of the form.

Now how do you do the nested form? It’s similar to normal database backed nested records.

Hope this helps, it is not documented anywhere I could find, and worked out mainly though reading the Rails source.

Rails makes it very easy to style fields with errors on your form. Unfortunately, the same error DIV with class ‘.field_with_errors’ is applied around labels, as well as inputs/checkboxes/selects. This tends to mess up the layout and double up on error display. To fix this, you can configure the field_error_proc in your application.rb to ignore labels. The code below calls the original error decoration proc for all types of tags except for labels.

Sometimes users manually edit query strings in the address bar, and make requests that have invalid encodings. Unfortunately Rails does not handle this neatly and the exception bubbles up. Eg,

ActionController::BadRequest
ActionView::Template::Error: Invalid query parameters: invalid %-encoding (%2Fsearch%2Fall%Forder%3Ddescending%26page%3D5%26sort%3Dcreated_at)

from:
/rack/lib/rack/utils.rb:127:in `rescue in parse_nested_query'

[Note: This was with Passenger, which passed the request through to the app – your mileage may vary with other servers]

In the case of my app, these corrupted query strings are not that important, but users are receiving 500 server error pages. Sometimes they end up with a bad query string URL cached in browser history, so they keep going back to it rather than to the home page.

A simple solution, that gives a good user experience for my app, is to simply drop the query string on a request completely if it has invalid encoding. See my implementation using Rack middleware below:

Say you’ve got a few app servers, and you want to serve up some largish files from your rails app (eg, pdfs) behind a login screen. Well, you could put them on s3 and redirect the user to s3 with expiring links. However, this would mean the eventual URL the user gets in their browser is going to be a s3 URL that expires in an ugly way with an XML error when the link expires. And if the link (copied from the address bar) is shared around, it’ll work for non-authorised people for a little while. Then when the s3 link expires, the receivers of the link will never get to see your site/product (maybe they might want to register/subscribe), instead they’ll just get a yucky s3 API looking error in XML, and nowhere to go.

Well, where could we put the files? How about using the file system on the app servers?

Two things to solve.. how to efficiently ship the files to the app servers, and how to serve them without tying up expensive Ruby processes.

1. Shipping the files to the app servers with Capistrano and Rsync

If you’re using docker or similar, you might want to bake the files into the images, if there aren’t too huge.

Assuming you’ve got a more classic style of deploy though..

Welcome old friends Capistrano and Rsync. Using Rsync we can ensure we minimise time / data sending files using binary diffs. We can also do the file transfers simultaneously to app app servers using cap. Here’s the tasks I put together. The deploy_pdfs task will even set up the shared directory for us.

We’re sticking the files into the ‘shared/pdfs’ directory created by capistrano on the app servers. Locally, we have them sitting in a ‘pdfs’ directory in the root of the rails app. This might seem inconsistent (and it is), but the reason is due to limitations/security restrictions with X-Sendfile.

2. Serving the files with X-Sendfile/Apache to let Rails get on with other things

So Rails provides a helpful send_file method. Awesome! Just password protect a controller action and then send_file away! Oh but wait, that will tie up our expensive/heavy ruby processes sending files. Fine for Dev, but not so great for production. The good news is we can hand this work off to our lightweight Apache/nginx processes. Since I use Apache/Ubuntu, that’s what I’ll cover here, but the nginx setup is similar. Using the X-Sendfile header, our rails app can tell the web server a path to a file to send to the client.

How to set up Apache & Rails for X-Sendfile

Ok let’s get Apache rolling:

You need to whitelist the path that files can be sent from, and it can’t be a soft link. It needs to be an absolute path on disk. Hence we are using the ‘shared’ directory capistrano creates, rather than a soft linked directory in ‘current’. X-Sendfile header itself lets you send files without a path (just looks for the files in the whitelisted path), but unfortunately we can’t use this as Rails send_file checks the path exists and raises if it can’t find the file.

In your rails app in production.rb add:

  # Hand off to apache to send big files
  config.action_dispatch.x_sendfile_header = 'X-Sendfile'

In development you probably don’t need this since you won’t be using a server that supports x_sendfile. Without this config, rails will just read the file on disk and send it itself.

In a controller somewhere, just use send_file to hand off to Apache. You’ll need to specify the absolute path to the file in the ‘shared’ directory. I’d suggest putting the path to the shared directory in an environment variable or config file (however you do this usually for your app per environment), and then just append the relevant filename on to it. Also, remember to validate the requested filename (I use a whitelist of filenames to be sure), to avoid the possibility of malicious requests getting sent private files they shouldn’t from elsewhere on disk.

Rails 4.2.9 works well with Ruby 2.4.2 except for an incompatible change with invalid decimal values and String#to_d. BigDecimal was changed in 1.3.0 (which ships with Ruby 2.4) to throw an exception on invalid values passed to the constructor. This also impacted String#to_d and caused it to raise an exception when it didn’t previously.

If string_amount is an empty String, code like this:

string_amount.to_d

will throw an exception like:

ArgumentError:
  invalid value for BigDecimal(): ""

rather than returning 0 as it did on ruby 2.3 and below.

This is handled in Rails 5 but the change has not been ported back to Rails 4.

If you’re on Rails 4, you’ve got a few options. You could add a monkey patch in your application.rb or initializer:

class String
  def to_d
    begin
      BigDecimal(self)
    rescue ArgumentError
      BigDecimal(0)
    end
  end
end

Or alternatively upgrade the BigDecimal version in your bundle. Ie, add to your Gemfile:

gem "bigdecimal", ">= 1.3.2"

Bundler will ensure you get the later version of BigDecimal rather than the one that ships with Ruby 2.4, and the behaviour was later fixed in BigDecimal 1.3.2 (but Ruby does not include it yet). See the Ruby issue for more details.