There are at least 2 easy ways to unit test the validations of an ActiveRecord::Base subclass.

The most straight-forward way is to create a model and use the valid? method to determine if it is valid or not. For an example I'm going to reuse the PhoneNumber class I defined in a previous entry on ActiveRecord Unit Testing. The following code shows the three tests I've written to ensure that I've correctly defined a validates_presence_of for digits.

class PhoneNumberTest < Test::Unit::TestCase
  Column = ActiveRecord::ConnectionAdapters::Column
  
  test "digits are required to be valid" do
    PhoneNumber.stubs(:columns).returns([Column.new("digits", nil, "string", false)])
    number = PhoneNumber.new(:digits => "1234567890")
    assert_equal true, number.valid?
  end
  
  test "invalid with no digits " do
    PhoneNumber.stubs(:columns).returns([Column.new("digits", nil, "string", false)])
    number = PhoneNumber.new()
    assert_equal false, number.valid?
  end
  
  test "errors include digits on invalid digits" do
    PhoneNumber.stubs(:columns).returns([Column.new("digits", nil, "string", false)])
    number = PhoneNumber.new()
    number.valid?
    assert_equal "can't be blank", number.errors.on('digits')
  end

end

After adding the validates_presence_of call to the PhoneNumber class, all the above tests pass.

class PhoneNumber < ActiveRecord::Base
  validates_presence_of :digits
end

This all works fine, but I'm not sure it's necessary to actually cause a validation to occur just to test that one has been defined.

An alternative test implementation could mock the validates_presence_of call and load the file. If the validates_presence_of is defined correctly the class methods will be invoked.

class PhoneNumberTest < Test::Unit::TestCase
  Column = ActiveRecord::ConnectionAdapters::Column

  test "validates_presence_of is defined for digits" do
    PhoneNumber.expects(:validates_presence_of).with(:digits)
    load "#{RAILS_ROOT}/app/models/phone_number.rb"
  end

end

This method of testing doesn't require actually creating a PhoneNumber instance, instead it tests that the PhoneNumber validations are defined correctly.

The load call is a bit messy; however, we can clean that up by adding another method to ActiveRecord::Base.

class << ActiveRecord::Base
  def standard_path
    File.expand_path("#{RAILS_ROOT}/app/models/#{self.name.underscore}.rb")
  end
end

Following the above code change the test can be rewritten to the code shown below.

class PhoneNumberTest < Test::Unit::TestCase
  Column = ActiveRecord::ConnectionAdapters::Column

  test "validates_presence_of is defined for digits" do
    PhoneNumber.expects(:validates_presence_of).with(:digits)
    load PhoneNumber.standard_path
  end

end

Which implementation is better? Both implementations provide pros and cons. I suggest trying both out to determine which works best for you (or your team).