issue with check boxes

You know the helper for check boxes generates a hidden field with an
"unchecked" value.

In a form with fields like this:

  <% fields_for "project[invoice_attributes][]", invoice, :index =>
nil do |form| %>
    <%= form.check_box :paid %>
    ...

the parameters parser gets confused by the pair and puts the checked
value in a hash by itself, see this XML dump:

   http://pastie.org/216415

thus, there's a spurious hash on the one hand, and the actual model
hash gets the unchecked value unconditionally.

Looking at UrlEncodedPairParser#bind in request.rb, my understanding
is that we switch to a new hash when we get a key that exists in the
previous hash (top[-1]). Since at that point there's no knowledge
about what's a check box or boolean model attribute I don't quite see
how that could be addressed.

Ideas?

I can't think of a good server-side solution. One would somehow tell
Rails to infer missing boolean attributes, but looks difficult to come
with a proposal that is transparent and maintains backwards
compatibility.

In the client-side the best I can think of is to observe_field the
check box to insert/remove the hidden field as needed, but that would
need Prototype or some weird inline handler. Falling back to the
current behaviour if JavaScript is disabled. Ugh, doesn't sound right
either.

By now I've documented the issue:

http://github.com/lifo/docrails/commit/ea40dc09a564ebc2b0034e3c1ae2c4daffe7e79b