Answer by robe007 for Define the default value of a drop down menu from a class variable

According to the documentation about options_for_select, the second parameter is the selected value:

options_for_select(container, selected = nil)

So, the answer according to my question is:

<%= s.select :category, options_for_select(@categories, @categories.second) %>

And of course, you can also try with: options_from_collection_for_select

Note: Some credits also to: https://stackoverflow.com/a/2434434/2954267