[Haml] Add support for overriding the object_ref of an Object.

This is for overriding the default behavior of the object_ref function
which currently defaults to the underscored class name of the Object.

doc-src/HAML_REFERENCE.md

@@ -559,6 +559,29 @@ is compiled to:
       Hello!
     </div>
 
+If you require that the class be something other than the underscored
+object's class, you can implement the `haml_object_ref` method on the object.
+
+    # file: app/models/crazy_user.rb
+
+    class CrazyUser < ActiveRecord::Base
+      def haml_object_ref
+        "a_crazy_user"
+      end
+    end
+
+    -# file: app/views/users/show.haml
+
+    %div[@user]
+      Hello!
+
+is compiled to:
+
+    <div class='a_crazy_user' id='a_crazy_user_15'>
+      Hello!
+    </div>
+
+
 ## Doctype: `!!!`
 
 When describing HTML documents with Haml,

lib/haml/buffer.rb

@@ -278,7 +278,12 @@ RUBY
       ref = ref[0]
       # Let's make sure the value isn't nil. If it is, return the default Hash.
       return {} if ref.nil?
-      class_name = underscore(ref.class)
+      class_name =
+        if ref.respond_to?(:haml_object_ref)
+          ref.haml_object_ref
+        else
+          underscore(ref.class)
+        end
       id = "#{class_name}_#{ref.id || 'new'}"
       if prefix
         class_name = "#{ prefix }_#{ class_name}"

test/haml/engine_test.rb

@@ -69,6 +69,11 @@ class EngineTest < Test::Unit::TestCase
   }
 
   User = Struct.new('User', :id)
+  class CustomHamlClass < Struct.new(:id)
+    def haml_object_ref
+      "my_thing"
+    end
+  end
 
   def render(text, options = {}, &block)
     scope  = options.delete(:scope)  || Object.new
@@ -689,6 +694,12 @@ END
                  render("%p[user]{:style => 'width: 100px;'} New User", :locals => {:user => user}))
   end
 
+  def test_object_ref_with_custom_haml_class
+    custom = CustomHamlClass.new 42
+    assert_equal("<p class='my_thing' id='my_thing_42' style='width: 100px;'>My Thing</p>\n",
+                 render("%p[custom]{:style => 'width: 100px;'} My Thing", :locals => {:custom => custom}))
+  end
+
   def test_non_literal_attributes
     assert_equal("<p a1='foo' a2='bar' a3='baz' />\n",
                  render("%p{a2, a1, :a3 => 'baz'}/",