option_groups_from_collection_for_select with a ActiveRecord::Base single object

Rails documentation gives examples of using
option_groups_from_collection_for_select with two tables/objects.

How could it be done with one?

Example that almost works:

I have a form to enter address from both the United States and Canada.
It has the following:

<%= select_tag(
    "id")) %>

Just use options_from_collection_for_select


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Ok, that and some reading has gotten me to this:

<%= render(:partial => "states/state") %>

The only example that I've seen of this is in AWD Ch. 22 (p. 488
printed), and it seems to be a lot of work. The book wraps the
option_groups_from_collection_for_select in <select> tags (html, not a
helper). The first parameter given is the highest level array - in
your case, it would essentially be [ "Canada", "US" ] - but each value
in the array is another object (a class in the example) with the group
name and sub-options available through attr_reader methods.

It seems like a lot of work. If you want to do it the "good" way,
read more of the API on option_groups_from_collection_for_select (I
think that you were on the right track for what you want to do).
Since you only have two groups, you could also do a little hard-coding
and use helpers to get the states.