One of the simplest tests is to require a validation if, and only if, another condition is met. The :if parameter, available on every test, lets you define those conditions. (There’s a corresponding :unless parameter that works similarly but in the opposite direction.) The easiest way to use :if is to point it at a method that returns a boolean value. That way your code can stay readable, and you can put whatever complications are involved in the test into a more maintainable and testable separate method.

This example uses the value of the :can_send_email field to determine whether the :description field must have a value. Neither is a field that would typically need much validation, but they can easily be treated as connected:

# if person says 'can send email', then we'd like them to fill their
# description in, so we understand who it is we're sending mail to
validates_presence_of :description, :if => :require_description_presence?
  
# we define the supporting condition here
def require_description_presence?
  self.can_send_email
end

The validates_presence_of method will only perform its test if the condition specified by the :if parameter returns true. The :if parameter’s value comes from the require_description_presence? method, which in this case simply returns the value of can_send_mail.

While Rails’ built-in validation is very helpful for a broad range of data checking, there are always going to be times when it’s just not enough. For example, while Rails can check the length of a string in characters, it didn’t have a method that checks the length of a string in words until Rails 2.2 added validates_length_of.

Performing such checks requires two steps. First, you need to create a method called validate. ActiveRecord will always call the validate method if one is present. It’s best, however, not to perform your validations directly in that method. Instead, put calls to readily identifiable methods that contain your custom logic. To indicate that validation failed, use self.errors.add, as shown in Example 7-4. This will tell Rails that there is an error and which field it applies to, as well as give you a chance to add a message to the user.

def validate
    validate_description
end
  
def validate_description
  # only do this validation if description is provided
  unless self.description.blank? then
    # simple way of calculating words: split the text on whitespace
    num_words = self.description.split.length
    if num_words < 5 then
      self.errors.add(:description, "must be at least 5 words long")
    elsif num_words > 50 then
      self.errors.add(:description, "must be at most 50 words long")
    end
  end
end

When you perform validation this way, Rails does less work for you. The unless self.description.blank? line is necessary because you can’t just specify allow_nil => true. Similarly, there aren’t any automatically generated messages. You have to provide them. And finally, of course, you’re responsible for all of the validation logic itself.