assert_errors_on.rb

  # http://gist.github.com/49013
  #
  # assert_errors_on record, :email
  # assert_errors_on record, :email => 2
  # assert_errors_on record, :email => "is blank"
  # assert_errors_on record, :email => ["is blank", "is invalid"]
  # assert_errors_on record, :email, :plz => 2
  # assert_errors_on record, [:username, :email] => "is blank"
  def assert_errors_on(object, *attributes_or_hash)
    assert !object.valid?, "#{object.inspect} should be invalid."

    hash = attributes_or_hash[-1].is_a?(Hash) ? attributes_or_hash.pop : {}
    attributes_or_hash.each { |field| hash[field] = 1 }

    hash.each do |fields, count_or_messages|
      Array.wrap(fields).each do |field|
        errors = Array.wrap(object.errors.on(field))
        count_or_messages = Array.wrap(count_or_messages)  unless count_or_messages.is_a?(Fixnum)

        case count_or_messages
        when Fixnum
          assert_equal count_or_messages, errors.size,
            "#{count_or_messages} error(s) expected for #{object.class}.#{field} but got #{errors.inspect}."
        when Array
          assert_equal count_or_messages.sort, errors.sort,
            "#{count_or_messages.inspect} error(s) expected for #{object.class}.#{field} but got #{errors.inspect}."
        else
          fail "unknown type #{count_or_messages.inspect}"
        end
      end
    end
  end