Beyond Simple Declarations
The tests shown above are valuable, but also limited. They test a single value against a limited set of possibilities and donât allow interactions among different values. While Rails makes it easy to do easy things, it fortunately also make it fairly easy to do more complicated things. (You can find these more complicated examples in ch07/guestbook006.)
Test It Only If
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. (Thereâs 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 heredef require_description_presence?
self.can_send_email
end
The validates_presence_of
method will only perform its test ...
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