Capybara: Unable to find form field with js: true - ruby-on-rails-3

Question

I wrote some tests using capybara for request testing using poltergeist and phantomjs as javascript driver.
The following steps for filling in a login form works great without js:
it "signs in" do
visit new_user_session_path
fill_in "Email", with: user
fill_in "Password", with: password||"foobar"
click_button "Login"
end
If I define my test with js it "signs in", js: true do my test fails with error:
Capybara::ElementNotFound: Unable to find field "Email"
The login form itself is built using simple_form as form generator and bootstrap in frontend.
The fields do not have a label. the search text is only contained in the placeholder attribute.
= simple_form_for(resource, :as => resource_name, :url => session_path(resource_name)) do |f|
= f.input :email, :placeholder => User.human_attribute_name(:email), :label => false, :input_html => {:class => 'input-xlarge'}
= f.input :password, :placeholder => User.human_attribute_name(:password), :label => false, :input_html => {:class => 'input-xlarge'}
%div
= f.button :submit, "Login", :class => 'btn btn-large btn-primary'
This code generates the following html code
<form accept-charset="UTF-8" action="/users/login" class="simple_form new_user" id="new_user" method="post" novalidate="novalidate">
<div style="margin:0;padding:0;display:inline">
<input name="utf8" type="hidden" value="✓">
</div>
<div class="control-group email required user_email">
<div class="controls">
<input class="string email required input-xlarge" id="user_email" name="user[email]" placeholder="Email" size="50" type="email" value="">
</div>
</div>
<div class="control-group password required user_password">
<div class="controls">
<input class="password required input-xlarge" id="user_password" name="user[password]" placeholder="Password" size="50" type="password">
</div>
</div>
<div>
<input class="btn btn btn-large btn-primary" name="commit" type="submit" value="Login">
</div>
</form>
Do you have any Idea how to ensure that the fields are found even if js is activated?

Answer 1

I don't know how your Webrat test passed. In my experience Capybara can't find "Email" if there is no matching Label or id.
In your case, since you don't use label, I suggest you to find the field with id
fill_in "user_email", with user.email
# user_email is the id created by simple_form in general case. Verify yours.
# Don't need "#" before id.

Answer 2

Take a screenshot after visit new_user_session_path and verify html is being rendered when :js => true.
visit new_user_session_path
save_and_open_page
If nothing is being rendered, just an empty html document, make sure in your spec_helper.rb
config.use_transactional_fixtures = false
Also try using DatabaseCleaner gem.
RSpec.configure do |config|
config.use_transactional_fixtures = false
config.before(:suite) do
DatabaseCleaner.clean_with :truncation
end
config.before(:each) do
if example.metadata[:js]
DatabaseCleaner.strategy = :truncation
else
DatabaseCleaner.strategy = :transaction
end
DatabaseCleaner.start
end
config.after(:each) do
DatabaseCleaner.clean
end
end