Where did the div class=fieldWithError go? - ruby-on-rails

Question

I have the following simple form:
<% form_for(#weight) do |f| %>
<%= f.error_messages %>
<%= f.label :weight %>:
<%= f.text_field :weight, :size => 5 %> kg.
<%= f.submit "Add weight" %>
<%= f.error_message_on :weight %>
<% end %>
which displays a form of only one field: weight.
Normally it renders like this:
<form action="/weights" class="new_weight" id="new_weight" method="post">
<div style="margin:0;padding:0;display:inline"><input name="authenticity_token" type="hidden" value="jYoVJkDnv4a1DMGnelJpGPElbH0XWKPNlESTt9GvzdA=" /></div>
<label for="weight_weight">Weight</label>:
<input id="weight_weight" name="weight[weight]" size="5" type="text" /> kg.
<input id="weight_submit" name="commit" type="submit" value="Add weight" />
</form>
which is fine. When I submit this form without setting any weight I get a validation error. f.error_messages and f.error_messages_on :weight correctly display the error messages, but the label and text field are not surrounded in a div with the class fieldWithError as I normally expect in forms in Rails. I instead get this:
<form action="/weights" class="new_weight" id="new_weight" method="post">
<div style="margin:0;padding:0;display:inline"><input name="authenticity_token" type="hidden" value="jYoVJkDnv4a1DMGnelJpGPElbH0XWKPNlESTt9GvzdA=" /></div>
<div class="errorExplanation" id="errorExplanation">
<h2>1 error prohibited this weight from being saved</h2>
<p>There were problems with the following fields:</p>
<ul><li>Weight can't be blank</li></ul>
</div>
<label for="weight_weight">Weight</label>:
<input id="weight_weight" name="weight[weight]" size="5" type="text" /> kg.
<input id="weight_submit" name="commit" type="submit" value="Add weight" />
<div class="formError">can't be blank</div>
</form>
For reference, what I've should have gotten is this:
<form action="/weights" class="new_weight" id="new_weight" method="post">
<div style="margin:0;padding:0;display:inline"><input name="authenticity_token" type="hidden" value="jYoVJkDnv4a1DMGnelJpGPElbH0XWKPNlESTt9GvzdA=" /></div>
<div class="errorExplanation" id="errorExplanation">
<h2>1 error prohibited this weight from being saved</h2>
<p>There were problems with the following fields:</p>
<ul><li>Weight can't be blank</li></ul>
</div>
<div class="fieldWithErrors"><label for="weight_weight">Weight</label></div>:
<div class="fieldWithErrors"><input id="weight_weight" name="weight[weight]" size="5" type="text" /></div> kg.
<input id="weight_submit" name="commit" type="submit" value="Add weight" />
<div class="formError">can't be blank</div>
</form>
Any ideas why I don't get those divs? I have formtastic installed and it's in use in other forms, but as far as I know that shouldn't interfere with this form.
Update: just to be sure, I printed out debug(#weight), it has the errors:
--- &id002 !ruby/object:Weight
attributes:
created_at:
updated_at:
weight:
measured_on: &id001 !timestamp
at: "2009-11-22 01:30:13.522589 +01:00"
"#marshal_with_utc_coercion": false
user_id: 1
attributes_cache:
measured_on: *id001
changed_attributes:
measured_on:
user_id:
errors: !ruby/object:ActiveRecord::Errors
base: *id002
errors:
weight:
- !ruby/object:ActiveRecord::Error
attribute: :weight
base: *id002
message: :blank
options: {}
type: :blank
new_record: true
Update: the model is
class Weight < ActiveRecord::Base
belongs_to :user
validates_presence_of :weight, :measured_on
attr_accessible :weight, :measured_on
def after_initialize
self.measured_on ||= Time.now
end
end

Answer 1

This is a bug in Formtastic. It was fixed but it seems that at this moment no released version of Formtastic has the fix.
My own bug report is on http://github.com/justinfrench/formtastic/issues/closed/#issue/132
The fix can be seen on http://github.com/grimen/formtastic/commit/2b81d9af385dadf8b37dc14f387afe3d43e4958a
Ultimately the problem was using justinfrench-formtastic from github, which is outdated and abandoned instead of formtastic from gemcutter.

Answer 2

You may need to enclose your label & field in a block tag like p or div.
<p>
<%= f.label :weight %>:
<%= f.text_field :weight, :size => 5 %> kg.
</p>
This way rails has a place to slip the error back into, else it falls back to the form as it is doing now.