If you're using simple_form gem then you must note that it does automatically id your form elements, hence manually id-ing elements yourself is futile!
There is; however, a naming convention that simple_form follows to id form elements.
object_attribute
So if you're filling-in information for let's say a contact object as follows:
= simple_form_for @contact do |f|
= f.input :first_name
= f.input :last_name
simple_form will then automatically generate ids to each form element respectively as so:
id="contact_first_name"
id="contact_last_name"
Now you can simply make use of this naming convention to fulfill all your fill_in needs. Hope this helps.