How to get a Ruby substring of a Unicode string?

I have a field in my Rails model that has max length 255.

I'm importing data into it, and some times the imported data has a length > 255. I'm willing to simply chop it off so that I end up with the largest possible valid string that fits.

I originally tried to do field[0,255] in order to get this, but this will actually chop trailing Unicode right through a character. When I then go to save this into the database, it throws an error telling me I have an invalid character due to the character that's been halved or quartered.

What's the recommended way to chop off Unic开发者_开发问答ode characters to get them to fit in my space, without chopping up individual characters?

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Uh. Seems like truncate and friends like to play with chars, but not their little cousins bytes. Here's a quick answer for your problem, but I don't know if there's a more straighforward and elegant question I mean answer

def truncate_bytes(string, size)
  count = 0
  string.chars.take_while{|c| (a += c.bytes.to_a.length) <= size }.join
end

Give a look at the Chars class of ActiveSupport.

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Use the multibyte proxy method (mb_chars) before manipulating the string:

str.mb_chars[0,255]

See http://api.rubyonrails.org/classes/String.html#method-i-mb_chars.

Note that until Rails 2.1 the method was "chars".