Ruby metaprogramming to achieve dynamic methods?

This bit seems redundant:

@resource = {}
@voters = {}
@is_upvoted = {}

Since you're already looping an array to do your metaprogramming.

You might try something like:

class Foo 

  %w(
    resource
    voters
    is_upvoted
  ).each do |attr_sym|
    define_method attr_sym do |comment|
      instance_variable_set("@#{attr_sym}", {}) unless instance_variable_get("@#{attr_sym}")
      instance_variable_get("@#{attr_sym}")[comment.id]
    end
  end

end

Which I believe will give you methods roughly like:

class Foo 

  def resource(comment)
    @resource ||= {}
    @resource[comment.id]
  end

end

Personally, it seems not great to me to have comment.id in your method. Because what if someday you want to use a different attribute (or something else altogether) as the key?

So, I think I would do:

class Foo 

  %w(
    resource
    voters
    is_upvoted
  ).each do |attr_sym|
    define_method attr_sym do |key|
      instance_variable_set("@#{attr_sym}", {}) unless instance_variable_get("@#{attr_sym}")
      instance_variable_get("@#{attr_sym}")[key]
    end
  end

end

Now, it seems like you're going to want an easy way to set key-value pairs on your instance variable, so I guess I would try something like:

class Foo 

  %w(
    resource
    voters
    is_upvoted
  ).each do |attr_sym|
    define_method attr_sym do |key=nil|
      instance_variable_set("@#{attr_sym}", {}) unless instance_variable_get("@#{attr_sym}")
      hsh = instance_variable_get("@#{attr_sym}")
      return hsh[key] if key
      hsh
    end
  end

end

In which case you should be able to do (assuming you have a @comment variable that responds to id):

@comment.id 
 => 1 
foo = Foo.new
 => #<Foo:0x000056536d7504b0>
foo.resource 
 => {}
foo.resource[@comment.id] = :bar
 => :bar 
foo.resource 
 => {1=>:bar}
foo.resource[@comment.id]
 => :bar