gistfile1.rb

class Parent
  belongs_to :child, :autosave => true, :polymorphic => true
end

class SomethingController
  
  def create
    @parent = Parent.new(params[:parent])
    @parent.child = child
    if @parent.save
      # blah
    else
      # eff
    end
  end

private

  def child
    type = params[:parent][:child_type]
    type.new(params[type.underscore])
  end

end

<%= form_for @parent do |f| %>
  <%= f.text_field :child_type >
<% end =%>