This does not always return the correct result. For some input strings, it changes relative positions of the characters that are not in the ordering string.

def sort_string(str,ordering)
str.chars.sort_by { |c| ordering.index(c) || ordering.size}
end

p sort_string("bananapxcgsdfhjd", "anb").join("")
p "aaannbpxcgsdfhjd"

the sort_string method results in "aaannbcxp..." instead of "aaannbpxc...". I am running ruby 2.2.0p0