transient subclass of an ActiveRecord subclass

rubyonrails-talk
1

hey all

I posted this same topic yesterday and it never showed up on the group? I've heard reports of other peoples messages not showing up too... very annoying!

Anyway, I'd like to be able to have transient (no persistent data of their own) subclasses of my ActiveRecord classes but that seems to be problematic.

Basically I'd like to be able to do this...

class Film < ActiveRecord::Base ... ... end

class SomeTransient < Film ... ... end

class SomeOtherTransient < Film ... ... end

etc...

Then do this:

film = SomeTransient.find_by_title("some film title")

This just struck me actually... are ruby class methods not inheritable?

Clearly I could do all my stuff directly in the Film class but that would get messy since the film instances are going to be taking on many different personalities (lots of different derived attributes for each).

I'm not sure why this seems to be so difficult?

Thanks in advance! Tim

2

<snip>

Basically I'd like to be able to do this...

class Film < ActiveRecord::Base ... end

class SomeTransient < Film ... end

class SomeOtherTransient < Film ... end

etc...

Then do this:

film = SomeTransient.find_by_title("some film title")

This just struck me actually... are ruby class methods not inheritable?

Clearly I could do all my stuff directly in the Film class but that would get messy since the film instances are going to be taking on many different personalities (lots of different derived attributes for each).

I think you may be mixing up the semantics of inheritance here. Inheritance is a "is a kind of" relationship. Unfortunately, you haven't posted enough code to work out what exactly you are trying to do, but I suspect it might be a case for delegation (which is a "uses" relationship).

With inheritance, you are asserting that an instance of an inherited class "is a" SomeTransient, and is also a Film. Now, if I understand correctly, you want a Film to *behave* as a SomeTransient or SomeOtherTransient, depending on what it is being used for in your business logic.

Try:

class Film   attr_accessor :name end

require 'delegate' class SomeTransient < SimpleDelegator   def name_reverse     self.name.reverse   end end

You can now do:

f = Film.new f.name="Hello"

s = SomeTransient.new(f) s.name_reverse #=> "olleH"

Or in your case, s = SomeTransient.new( Film.find_by_title('some file title'))

The Delegate library takes care of delegating all methods that are not explicitly defined in the Delegate class to the delegate Film.

Cheers, Max

3

Thanks for the respones guys, sorry it took me so long to get back but "self.abstract_class = true" is, I think, what I was looking for:-)

Best Tim