Passing Arrays and Nested Params to url_for in Rails

Wed, 07 Feb 2007 13:12:00 -0800
ruby, rails, plugin, patch

A few weeks ago (okay, more than a few weeks ago, it took me a while to write this), I discussed the problems involved with passing nested hash parameters to named routes in Rails. My coding pair and I discovered another bug (still using rev 5522) when passing hash parameters to a named route in Rails, this time when the hash contains arrays. For example, consider the following call to a named route:

person_url(:name => ['Ryan', 'Kinderman'])

In order for the params hash to get decoded properly on the server, the resulting URL must be encoded to look like this:
"http://someurl.com/people?name[]=Ryan&name[]=Kinderman"
Unfortunately, it gets encoded to look like this:
"http://someurl.com/people?name=Ryan%2FKinderman"
For those of you unfamiliar with CGI escaping, the %2F translates into the '/' character. So, you end up with a params hash in the controller where params[:name] == ['Ryan/Kinderman']. How disappointing. To get around this in the past, I've chosen to either split the hash value on '/', or use my own encoding of arrays that Rails can handle, and then simply decode them myself within the controller. In the above example, I could have done something like:

person_url(:name => {0 => 'Ryan', 1 => 'Kinderman})

Of course, without the patch I described a few weeks ago, this kind of thing would not be possible either, because Rails can't encode nested hash parameters.

What I present here is a detailed explanation of the problem, with instructions at the end on how to install my plugin patch to fix it. My explanation and patch address the issues for both nested and array parameters. There are a number of methods involved in the solution to this problem. It may be useful at this point for you to refer to Jamis Buck's excellent articles on the gory details of Rails route recognition and generation.

When you call link_to or url_for, either explicitly, either explicitly or through the named route *_url and *_path methods, they roughly follow the following call sequence for processing route parameters:

ActionController::Base.url_for in action_controller/base.rb
OR
ActionView::Helpers::UrlHelper#link_to in action_view/helpers/url_helper.rb

RouteSet#generate in action_controller/routing.rb

RouteSet#options_as_params in action_controller/routing.rb

Array#to_param in active_support/core_ext/array/conversions.rb

Route#generate in action_controller/routing.rb

Route#write_generation in action_controller/routing.rb

Route#append_query_string in action_controller/routing.rb

Route#build_query_string in action_controller/routing.rb

The problems start in the call to options_as_params. This method is not recursive, and processing nested parameters is a recursive problem. The next issue with options_as_params is not actually in the method, but in the to_param method that it calls. If you look at the Rails implementation of Array#to_param, you'll see that all it's doing is joining the elements into a '/' separated string. This doesn't get processed back into separate array elements when the request is received by the controller. So, in the case when value is an Array instance during a call to options_as_params, the resulting string is encoded incorrectly.

The other specific issue lies in the Route#build_query_string method. Take a look at the method, and notice the part that looks like:

if value.class == Array
  key <<  '[]'
else
  value = [ value ]
end

The check for the Array class causes a problem when passing an array to url_for as an option parameter when that array comes from the params hash from within a controller action (*whew*, that was a mouthful!). This is because what you thought was an array is actually an instance of ActionController::Routing::PathSegment::Result. To be honest, I don't know why this is happening. I looked at the code and realized that it'd take me longer to figure out than what I wanted to spend at the time. However, if someone could explain it to me, I'd love to hear it. In any case, to solve this particular problem, the conditional needs to be changed from a check for only Array to Array and any subclasses using something like the is_a? method.

So, those are the issues involved in why array and nested hash parameters don't work properly in calls to url_for. Rather than going through my solution, I'm offering it as a Rails plugin with full unit test coverage, and plan to submit it as an actual patch to the Rails team, with the code cleaned up a bit more. Maybe there are reasons why this sort of thing isn't supported, but I can't think what they might be. I'll post updates here if and when I get more information on this. If you have comments or questions on this patch or parts of the code, please let me know.

You can install the Rails plugin by typing the following into your command-line: ruby script/plugin install git://github.com/ryankinderman/nested_params_patch.git To see the issues I've discussed first-hand, after installing the plugin, take a look at controller_test.rb.

Addendum: I checked, and as of revision 6141 of Rails, the issues covered by this article are still present, and the plugin still fixes them.

Addendum (2007/04/03): I've just got around to confirming that, as rwd's commented, the bug has been fixed. If you're using revision 6343 or later of Rails, you probably aren't going to need this patch. Yay!