truncate text after exceeding certain size - ruby-on-rails

Question

Rails offers the truncate method when you want to truncate text that exceeds a certain character length. This is the example provided here:
truncate("Once upon a time in a world far far away")
# => "Once upon a time in a world..."
It truncates a given text after a given :length if text is longer than :length
But I have the following example:
str = "abcdefhijklmno"
After 12 characters I want it truncated, so the above text should look like this:
abcdefhijklm...
But I tried using the truncate method and cannot get the desired result:
> str = "abcdefhijklmno"
=> "abcdefhijklmno"
> str.truncate(15)
=> "abcdefhijklmno"
> str.truncate(14)
=> "abcdefhijklmno"
> str.truncate(13)
=> "abcdefhijk..."
> str.truncate(12)
=> "abcdefhij..."
I would think that truncate(12) would do it but it truncates after 9 characters. What am I doing wrong?

Answer 1

argument of truncate means size of output string (with "..."):
str.truncate(13)
=> "abcdefhijk..."
str.truncate(13).size
=> 13
You can change default omission(...) by empty space if you want:
str.truncate(13, omission: '')
=> "abcdefhijklmn"
More about Rails String#truncate here.

Answer 2

I don't believe str.truncate will do what you want out of the box. But since it's really just an extra if statement, it's easy to write:
def trunc(str, length)
addition = str.length > length ? '...' : ''
"#{str.truncate(length, omission: '')}#{addition}"
end
Or slightly simplified, and without a Rails dependency (as mentioned by Ilya in the comments):
def trunc(str, len)
"#{str.first(len)}#{'...' if str.size > len}"
end
And the test cases:
2.2.1 :005 > s = 'abcdefghijklmno'
=> "abcdefghijklmno"
2.2.1 :006 > trunc(s, 20)
=> "abcdefghijklmno"
2.2.1 :007 > trunc(s, 15)
=> "abcdefghijklmno"
2.2.1 :008 > trunc(s, 14)
=> "abcdefghijklmn..."
2.2.1 :009 > trunc(s, 13)
=> "abcdefghijklm..."
2.2.1 :010 > trunc(s, 0)
=> "..."
2.2.1 :011 > trunc(s, 1)
=> "a..."