module Shoulda::ActiveRecord::Matchers

it { should allow_value("isbn 1 2345 6789 0").for(:isbn) }
it { should_not allow_value('bad').for(:isbn) }
Example:

translation for :invalid.
errors.on(:attribute). Regexp or string. Defaults to the
* with_message - value the test expects to find in
Options:

Ensures that the attribute can be set to the given value.

def allow_value(value)
  AllowValueMatcher.new(value)
end