Thomas Lee

The other day I was discussing Rails’ form processing behavior with Ben, when the topic of editing multiple associations in a single form came up. Effectively, he needed to be able to manipulate a collection of records associated with a parent via a has_many association. For each item in this collection, he wanted something like two drop down lists — nothing unreasonable. He said it was similar to recipe 13 from Advanced Rails Recipes — “Handle Multiple Models in One Form” — except that he had to use two fields instead of the one textfield, and it was proving to be surprisingly tough.

I know what you’re thinking: surely this can be extended to more than one form field per item without any problem? Up until I read the code myself a few months ago, I wouldn’t have thought it would be so difficult either — but it’s true. The parameter parsing code is quite quirky and, without reading the detail of the source, borderline unpredictable. I’ve had a few ideas for dealing with more complicated forms in Rails ever since I first ran into this issue. Only recently have I actually gotten around to doing something about it. Here’s an abridged version of an email I just sent to the rails-core mailing list:

Hi,

…

A colleague of mine recently ran into a related problem when dealing with collections of records in Rails forms, which prompted me to finally do something about the parameter parsing behavior. Now, while I’m sure we’re not the only ones hitting this wall, I’m aware that proposals to change to the parameter parsing semantics in Rails is likely to be met with a little caution, hesitation — possibly even terror. With that in mind, I’ve put up a plugin so everybody can give this a go without having to apply any nasty patches:

http://www.vector-seven.com/git/rails/plugins/form_collections.git

Please see the README (attached) for the full details.

…

Cheers,
Tom

This is a plugin which greatly simplifies the existing Rails parameter parsing code, and makes it a whole lot easier to deal with collections in your forms. I encourage all you Rails developers out there to take a look when you get the opportunity, I’d love to hear your opinions on this one. You can get a copy of the git repository using the following command:

$ git clone http://www.vector-seven.com/git/rails/plugins/form_collections.git

UPDATE: Here’s the contents of the README file:

> FormCollections
> ===============
>
> This plugin effectively rewrites UrlEncodedPairParser and provides the
> following features/changes:
>
> 1. The parameter parsing algorithm is much simpler
>
> There's all sorts of wacky stuff happening in the current implementation of
> UrlEncodedPairParser#parse. This plugin does away with that. A stack is no
> longer used during the parse. Parsing an Array vs. parsing a Hash is no longer
> all that different.  See lib/patch_url_encoded_pair_parser.rb.
>
> 2. Parsing "a[b][0][c]=6" yields {"a" => {"b" => [{"c" => "6"}]}}
>
> Previously this would be parsed to: {"a" => {"b" => {"0" => {"c" => "6"}}}}
>
> In other words, parameters that were formerly treated as hashes with a numeric
> index are now actually arrays.
>
> This is important, because the order in which form fields are present may be
> important to the back-end processing code. Here, the array in "b" will
> preserve the ordering specified -- generating an Array rather than a Hash for
> numeric indices. Most other scenarios should essentially work as before, with
> the exception of a few error cases (e.g. parsing "a/b@[c][d[e][]=f" yields
> {"a/b@" => {"c" => {"d[e" => ["f"]}}} instead of {"a/b@" => {"c" => {}}}).
>
> The main problem here is that if an array is instantiated with a value at
> index 1000000, then we'll have 999999 nil elements. I figure this can easily
> be worked around with an Array-like structure that consumes only as much memory
> as it needs but otherwise acts as an Array, or a hard limit set via
> configuration variable.
>
> 3. fields_for now treats Arrays properly ...
>
> Example:
>
>  <% fields_for @post.comments do |comment_form, comment| %>
>    <%= comment.new_record? ? "" : comment_form.hidden_field(:id) %>
>    <%= comment_form.hidden_field :post_id %>
>    <%= comment_form.text_field :content %>
>    <hr />
>  <% end %>
>
> This will generate the following HTML for an array with two elements (one a new_record?, the other existing):
>
>  <input id="comments_0_id" name="comments[0][id]" type="hidden" value="1" />
>  <input id="comments_0_post_id" name="comments[0][post_id]" type="hidden" value="1" />
>  <input id="comments_0_content" name="comments[0][content]" size="30" type="text" value="a test comment" />
>  <hr />
>  <input id="comments_1_id" name="comments[1][post_id]" type="hidden" value="1" />
>  <input id="comments_1_content" name="comments[1][content]" size="30" type="text" value="" />
>  <hr />
>
>
> Example
> =======
>
> Just install the plugin, and the new behavior should already be in effect.
>
> Copyright (c) 2008 Thomas Lee ..., released under the MIT license
>