`it do` is something I occasionally do for quick checks, but usually try to take out (possibly because I heard it as a regular speech pattern in a county in which I used to live. I'll leave it as an exercise for the reader to work out which)

How about something like:

```ruby
    it 'constrains the length of all URL fields' do
      too_long_url = 'http://'.ljust(65536), 'x' 

      [:new_url, :suggested_url, :archive_url].each do |url_attr|
        build :mapping, url_attr => too_long_url
        mapping.should_not_be_valid
        mapping.errors[url_attr].should include('is too long')
      end
    end
```