fields_for: how many objects can it handle - max string size?

Our app is getting a "string sizes too big" error in an .erb file that
is trying to render a large block of checkboxes that represent color
attributes that belong_to the product being edited in this view. The
code worked fine when we had a small number of colors -- now that the
number of colors is 750+, we get an error on the fields_for line (see
below).

Since this sounds somewhat like some older memory leak issues, I
confirmed that our host, Engine Yard, does not have an old Ruby
version (they are on 1.8.6 patch level 287). The same error occurs on
Mac OS/X with Ruby 1.8.6 patch 369.

I've had no luck searching for that error with "fields_for" or finding
any docs on size limitations inherent to fields_for.

Processing Admin::ProductsController#edit (for 173.45.210.193 at
2010-04-29 05:21:30) [GET]
  Parameters: {"id"=>"17"}
Rendering template within admin/layouts/base
Rendering admin/products/edit

ActionView::TemplateError (string sizes too big) on line #67 of app/
views/admin/products/edit.html.erb:
64:
65: <div id="colorCheckboxes"> <!-- event handler searches within
this div to find checkboxes -->
66: <% previousObject = nil -%>
67: <% f.fields_for :prod_method_colors do |pmc| -%>
68: <% pmcObj = pmc.object %>
69: <%= "<br/><br/> Colors for
#{PRINT_METHOD_TEXT[pmcObj.print_method]} <br/>" if previousObject ==
nil || (pmcObj.print_method != previousObject.print_method) %>
70: <% previousObject = pmcObj -%>

There's only one place in the ruby C code I could find that raises
'string sizes too big'. It's raised in a string concatenation function
if the length of the existing string plus the length of what you're
concatenating exceeds LONG_MAX, ie 2^31-1 on a 32bit system. I'm not
sure why fields for would be attempting to concatenate two strings
with a combined length of 2GB.

Fred