On Tue, 29 Nov 2005, Kris wrote:
Yet Another Ruby N. (YARN) question.
Isn’t Ruby N. redundant? But anyway, welcome.
I see Rails makes extensive use of class-declarations.
class LineItem < ActiveRecord::Base
belongs_to is a class-declaration in the above code. Coming from a C
It’s not a declaration; it’s a call to a method, specifically a class
method of ActiveRecord::Base. (Actually, because of the way the
ActiveRecord code base is engineered, it’s sort of a
pseudo-class-method, but I strongly advise you not to worry about
that For all practical purposes it’s a class method.)
A class method is a method defined directly on a class object. Here’s
a simple example:
C.hello # hi
Note that the class method C.hello is completely unrelated to C’s
instance methods. In fact, you could have an instance method with the
same name, with no conflict:
puts “hi from instance!”
C.hello # hi
C.new.hello # hi from instance!
family of languages I could not grasp this initially. When does this
code get executed - does it happen when the class is loaded for the
first time or every time a new instance of the class is created?
It happens when the class definition itself is executed – which will
be when the file is loaded.
In this case I think the class-declaration code(belongs_to) looks at
the corresponding database table structure and injects attributes and
methods into the LineItem class. Am I right?
Yes. The route to this is somewhat circuitous, but basically an
association method will trigger the matching up of table names,
thing_id fields, etc., and dynamically create the methods that allow
the object access to the associated class.
I tried this in irb and got an error. Why doesn’t this work? In Rails I
think mixins are used to provide this kind of functionality.
It doesn’t work because hello and bye are both instance methods – but
by calling hello in the definition scope of D, you’re treating it like
a class method. You’re basically saying: send the message “hello” to
the class object D, which doesn’t do anything because D doesn’t
respond to that message.
To me these seem to be idioms of the language. Is there a site where
you could find gems like these?
I’m not quite sure what you mean… but I hope the above is helpful