How to make a check_box checked in rails?

I made checkboxes using the following rails form helper:

<%= check_box("tag", tag.id) %>

However, I need to make some of them checked by default. The rails documentation doesn't specify how to do this. Is there a way? How?

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.