Collection Associations Build Method Not Supported for STI

The belongs_to association build method does not currently allow for specifying a :type subclass. This creates a lot of confusion and annoyances around STI support.

# Currently working...
u = User.first
b = u.badges << Badges::Superhero.new
# b.class == Badges::Superhero

# Not working...
u = User.first
u.badges.build(:type => Badges::Superhero)
# b.class == Badge

The quick fix is to change the build_association method in ActiveRecord::Reflection::AssociationReflection to detect if :type is a Class object and create the appropriate subclass model.

class ActiveRecord::Reflection::AssociationReflection
  def build_association(*opts)
    col = klass.inheritance_column.to_sym
    if (h = opts.first).is_a? Hash and (type = h.symbolize_keys[col]) and type.class == Class
      opts.first[col].to_s.constantize.new(*opts)
    elsif klass.abstract_class?
      raise "#{klass.to_s} is an abstract class and can not be directly instantiated"
    else
      klass.new(*opts)
    end
  end
end

Full writeup here: http://www.simple10.com/rails-3-sti/