How to make a check_box checked in rails?

If you need the check_box to be checked on new, and correctly filled on edit you can do:

<%= f.check_box :subscribe, checked: @event.new_record? || f.object.subscribe? %>

As I mentioned here

Here's how to do it as of rails 4, I didn't check older documentation.

<%= check_box("tag", tag.id, {checked: true}) %>

This will make the checkbox checked. Of course instead of true you will put in some logic which determines if each one is checked.