Initializing attributes by default

[joevandyk]

Is it possible to get virtus to do something like:

require 'virtus'                                                                                    

class Address                                                                                       
  include Virtus                                                                                    
  attribute :street,      String                                                                    
  attribute :city,  String                                                                          
end                                                                                                 

class User                                                                                          
  include Virtus                                                                                    
  attribute :name, String                                                                           
  attribute :address, Address                                                                       
end                                                                                                 

u = User.new                                                                                        
fail unless "" == u.name                                                                            
fail unless "" == u.address.city

I'm building a form handling library based on virtus, and not having to deal with nil's would make this a lot easier.

[dkubb]

What about specifying the default as an empty string, eg:

attribute :name, String, :default => ''

[joevandyk]

Hm, so
attribute :address, Address, default => Address.new
?

[dkubb]

You could do this too:

attribute :address, Address, :default => proc { Address.new }

Although I wouldn't be surprised if your approach works too. Virtus will dup the default object if it can, so that anything specified as a default won't be a singleton and shared across multiple instances.

[senny]

@dkubb if ´Addressewas aVirtus::ValueObject` wouldn't it make sense to reference the same in-memory object since it can't be modified anyways?

[dkubb]

@senny I wonder if we should make it so value objects are also Singleton instances? That way there is only ever one copy in memory at a time. That way we're leveraging something from stdlib rather than inventing our own semantics for the objects.

[dkubb]

oh wait, nevermind that won't work. Singleton instances are 1:1 with the class names, while value objects only need to have one instance per distinct value.

[dkubb]

@senny so yeah, to answer your question, yes, I think we should do that :) What I'm thinking is we should make it so that #dup and #clone return self so that inside Virtus::Attribute::DefaultValue when it clones the object it just gets back the same instance. For a value object in other contexts I think that's valid behaviour.

[joevandyk]

@dkubb Thanks! This is what I've got so far for my form library. https://img.skitch.com/20120209-pewtuuecpkubktpnts2iinbbu1.png (same idea as https://github.com/joshsusser/informal, but better)

[senny]

@solnic I think this can be closed.