Extract one page of PDF with rmagick?

I know this works from the command line:

% convert ‘svn-book.pdf[0]’ first-page.png

The above example will only process the first page of a PDF. This is
great for creating a thumbnail of the ‘cover page’ of a PDF, and it
avoids the CPU load required to process a huge file. (in this case,
the svn-book.pdf is 300 pages)

How would I accomplish this using the magick_file_column?

To use the command line as an example, the ‘svn-book.pdf[0]’ isn’t
really and argument such as -size 120x120, etc. How can I pass that
[0] to rmagick when I’m processing a PDF?