Capybara: How do I fill in a input field by its ID

I have this:

<input class="string optional" 
    id="course_group_courses_attributes_2_name" 
    name="course_group[courses_attributes][2][name]" 
    placeholder="Lengua" 
    size="15" 
    type="text" />
</div> 

How do I fill in a that field by its ID?

fill_in accepts the id as first parameter:

fill_in 'course_group_courses_attributes_2_name', :with => 'some text'

You can also do it with:

find('Id or class here').set('some text')

fill_in accepts id without the # symbol which I found confusing at first. Also note that if your input field is in a modal, you may need to wait for the transition or fade in before the field can be filled in.