How to implement an abstract class in ruby?

Just to chime in late here, I think that there's no reason to stop somebody from instantiating the abstract class, especially because they can add methods to it on the fly.

Duck-typing languages, like Ruby, use the presence/absence or behavior of methods at runtime to determine whether they should be called or not. Therefore your question, as it applies to an abstract method, makes sense

def get_db_name
   raise 'this method should be overriden and return the db name'
end

and that should be about the end of the story. The only reason to use abstract classes in Java is to insist that certain methods get "filled-in" while others have their behavior in the abstract class. In a duck-typing language, the focus is on methods, not on classes/types, so you should move your worries to that level.

In your question, you're basically trying to recreate the abstract keyword from Java, which is a code-smell for doing Java in Ruby.

I don't like using abstract classes in Ruby (there's almost always a better way). If you really think it's the best technique for the situation though, you can use the following snippet to be more declarative about which methods are abstract:

module Abstract
  def abstract_methods(*args)
    args.each do |name|
      class_eval(<<-END, __FILE__, __LINE__)
        def #{name}(*args)
          raise NotImplementedError.new("You must implement #{name}.")
        end
      END
      # important that this END is capitalized, since it marks the end of <<-END
    end
  end
end

require 'rubygems'
require 'rspec'

describe "abstract methods" do
  before(:each) do
    @klass = Class.new do
      extend Abstract

      abstract_methods :foo, :bar
    end
  end

  it "raises NoMethodError" do
    proc {
      @klass.new.foo
    }.should raise_error(NoMethodError)
  end

  it "can be overridden" do
    subclass = Class.new(@klass) do
      def foo
        :overridden
      end
    end

    subclass.new.foo.should == :overridden
  end
end

Basically, you just call abstract_methods with the list of methods that are abstract, and when they get called by an instance of the abstract class, a NotImplementedError exception will be raised.