From: Dave Thomas <dave@...>
Date: 2003-06-28T01:38:47+09:00
Subject: Re: Magic at class-definition time

Clifford Heath wrote:


> There's a few things I don't get about Dave's example though, so
> perhaps he or someone else would explain?



> 
> I'm guessing that "field" defines methods for the current class
> instance. In this case does "field" have to eval a string with
> the method definition in it or is there a better way?

That's how I did it:

   def Table.field(type, name, *options)
     f = Field.new(name, type, options)

     @fields << f

     @primary_key = f if f.primary_key?

     create_accessors_for(f)
     create_writer_for(f)
   end

   def Table.create_accessors_for(field)
     name = field.name
     class_eval <<-EOS
     def #{name}
       @#{name}
     end
     def #{field.setter_name}(val)
       @#{name} = val#{field.type.conversion_function}
     end
     EOS
   end

   def Table.create_writer_for(field)
     name = field.name
     class_eval <<-EOS
       def #{name}=(val)
         set_changed(:#{name}) if val != @#{name}
         @#{name} = val
       end
     EOS
   end

> 
> Why did Dave call "field" from a block attached to "table", instead
> of just from the class RegionTable?

It gives me some encapsulation of the generation of the table-specific
stuff, and it seemed neat at the time. For example, more complex table
classes have extra code in there:

   class StateNameTable < Table
     Suspended   = 'SUSPND'
     WaitPayment = 'WTPAY'
     Active      = 'ACTIVE'

     table "state_name" do
       field char(6),       :stt_id,            pk
       field varchar(30),   :stt_desc

       initial_values([Suspended,   "Inactive"],
                      [WaitPayment, "Not paid"],
                      [Active,      "Active"]
                      )
     end
   end

In the end, I could do it without the yield, but I like the way that
doing it allows me to know when the definition is finished, and hence
generate all the accessors.

I can see I'm going to have to write this up at some point... :)

Cheers



Dave