Issue with has_many and Array.empty?

I've posted a narrative of my issue on the Google Rails Group:

/app/controllers/document_groups_controller.rb
  def show
    @document_group = DocumentGroup.find(params[:id])
    @journal = Journal.find(params[:journal_id])
    @document_group_templates = @document_group.document_templates
    @logger = RAILS_DEFAULT_LOGGER
    
    #
    # this is the money line: comment it out and @document_group_templates.empty?
    # is true in the view.  leave it in, and it's false.  true is correct.
    #
    @logger.debug "###### --->" + @document_group_templates.inspect
    #
    # it turns out that this triggers the SQL to be run by Rails that acutally
    # populates the @document_group_templates Array.
    #

    render :action => :show, :layout => 'ajax_div'
  end


/app/views/document_groups/show.rhtml
#
#@document_group_templates.empty? evaluates to true without the inspection above
#
<% if !@document_group_templates.empty? %>
# 
# now, what's interesting is that if i change this to 
#
#  if @document_group_templates.count > 0
#
# that also triggers the SQL to be run.  but, interestingly, #size does not 
# trigger the SQL.
#
 <ul>
 <% 
  @document_group_templates.each do |child| 
            %><li><%= link_to child.name, :controller => :template, :action => :show, :id => child.id %><%
  end
  %>
  </ul>
<% end %>

/app/models/document_group.rb
class DocumentGroup < ActiveRecord::Base
  has_many :document_templates, :order => :document_group_position
  acts_as_tree :counter_cache => true
end

/app/models/document_template.rb
class DocumentTemplate < ActiveRecord::Base
  belongs_to :document_group, :counter_cache => true
  acts_as_list :scope => :document_group, :column => :document_group_position
end