class Shoulda::ActiveRecord::Matchers::ValidateUniquenessOfMatcher

def validate_after_scope_change

could actually find all values for scope and create a unique
to a value that's already taken. An alternative implementation
TODO: There is a chance that we could change the scoped field 
def validate_after_scope_change
  if @scopes.blank?
    true
  else
    @scopes.all? do |scope|
      previous_value = @existing.send(scope)
      # Assume the scope is a foreign key if the field is nil
      previous_value ||= 0
      next_value = previous_value.next
      @subject.send("#{scope}=", next_value)
      if allows_value_of(existing_value, @expected_message)
        @negative_failure_message << 
          " (with different value of #{scope})"
        true
      else
        @failure_message << " (with different value of #{scope})"
        false
      end
    end
  end
end