Where did the div class=fieldWithError go?

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

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.

Answer 2

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.