metaprogramming, define_method, and class_eval issues

rubyonrails-talk
1

I have a 2-part question here that's a little tricky and starting to make my head hurt.

I'm working on a plugin for personal use, where I have an rclients table (not named clients because of conflicts with another plugin), and several other models that can have Rclients (polymorphic association). I've defined a method acts_as_client_entity that will set up the necessary associations.

Let me first give my functioning code:

############### join model: class ClientEntityAssociation < ActiveRecord::Base   belongs_to :rclient   belongs_to :entity, :polymorphic=>true end

################ plugin: module ClientEntity   module ClassMethods       def acts_as_client_entity

has_many :client_entity_associations, :as=>:entity, :dependent=>:destroy

has_many :rclients, :through=> :client_entity_associations

has_one :primary_client, :through=> :client_entity_associations,                                                :source => :rclient, :conditions=>["client_entity_associations.primary = ?",true]

        klass = self.name.tableize         Rclient.class_eval "has_many :#{klass}, :through=>:client_entity_associations"

        define_method("primary_client=") do |rclient|           client_entity_associations.update_all("`primary` = false")           assoc = client_entity_associations.find_by_rclient_id(rclient.id)           if assoc && !assoc.primary             assoc.update_attribute :primary,true           else

client_entity_associations.create(:rclient_id=>rclient.id,:primary=>true)           end         end       end   end end

2

The first part of my question is whether there is a way to pass in a

reference to the calling class (klass) without first setting it as a

variable. My understand is that if I used self directly in the

class_eval statement, it would evaluate to Rclient.

You should use the included callback which includes the class it’s being included into

http://ruby-doc.org/core/classes/Module.html#M001660

So your acts_as_client_entity should include the module in to the current class, then you should do your initialisation in the included method.

The second part of my questions is why I had to use define_method for

primary_client= (as opposed to def primary_client=). I kept getting

conflicts with the dynamic methods rails created from the statement

has_one :primary_client . . .

I would just like to know why define_method seems to be a little more

forceful.

No idea, sorry.

Cheers,

Andy

3

The first part of my question is whether there is a way to pass in a reference to the calling class (klass) without first setting it as a variable. My understand is that if I used self directly in the class_eval statement, it would evaluate to Rclient.

Maybe, don't know off hand.

The second part of my questions is why I had to use define_method for primary_client= (as opposed to def primary_client=). I kept getting conflicts with the dynamic methods rails created from the statement

I *think* the def method is scoping it inside the proc whereas define_method is actually evaluating it within the context of the class_eval (and therefore getting defined as an instance method of the class).

I highly recommend the O'Reilly book, The Ruby Programming Language since it gives details about metaprogramming that's rarely found elsewhere and organized in the same place. There are stuff about how to access the variable bindings and may answer your first question as well.

Ho-Sheng Hsiao http://hosheng.blogspot.com