RSpec does not recognise my internationalised labels - ruby-on-rails

Question

In my Rails 4 application I have specified my locales as follows:
config/locales/views/show_user/en.yml
with content:
en:
activerecord:
attributes:
user:
first_name: First name
last_name: Last name
...
layouts:
header:
home: "Home"
help: "Help"
...
footer:
about: "About"
contact: "Contact"
users:
new:
signup: "Sign up!"
create_my_account: "Create my account"
When I run my application in my localhost, all my labels display correctly. However, my RSpec test,
describe "with valid information" do
before do
fill_in "First name", with: "Jenny"
fill_in "Last name", with: "Johnson"
fails when I make the following change to my partial, /views/users/_fields.html.erb
<%= f.label t :first_name, scope: [:activerecord, :attributes, :user] %>
<% # f.label :first_name %>
<%= f.text_field :first_name %>
<%= f.label :last_name %>
<%= f.text_field :last_name %>
My RSpec test displays this failure:
1) User pages signup with valid information should create a user
Failure/Error: fill_in "First name", with: "Jenny"
Capybara::ElementNotFound:
Unable to find field "First name"
# ./spec/requests/user_pages_spec.rb:88:in `block (4 levels) in <top (required)>'
I have noticed that the generated html of my form displays a different id for first_name, but I do not know if this is important:
<label for="user_First name">First name</label>
<input id="user_first_name" name="user[first_name]" type="text" />
<label for="user_last_name">Last name</label>
<input id="user_last_name" name="user[last_name]" type="text" />
My question is therefore: Why can Capybara not find my label, :first_name, while my page displays correctly in my browser?

Answer 1

It is important. Capybara uses the locator argument to fill_in in one several ways:
if there is a text field (or similar input) with that id then it will use that field
if there is a label that contains the text, then it will use the field that label for.
You're in the second case, so capybara uses the for attribute on the label to find the text field. This doesn't work because the label's for is user_First name but the id on the text field is user_first_name.
By default rails derives both the label text and the for attribute of the label from the value you pass as the first argument, so you should pass the attribute name as the first argument and then the human readable translation as the second argument
f.label :first_name, t(:first_name, scope: [:activerecord, :attributes, :user])
My reading of the documentation is that looking for this transaction would actually be the default - you may not need to specify it at all.