Perform for loop using eco with increment and metadata usage - coffeescript

Question

I'm using docpad and on index page, in navigation pane I want to get a list of links grouped by category.
Category is defined in each markdown document in meta info on top. For example category:"tutorials"
So I have this:
<% for docu in #getFilesAtPath("document-repository").toJSON(): %>
<li><h2><%=cat=docu.category%></h2></li>
<%for docu in #getFilesAtPath("document-repository",category:cat}).toJSON():%>
<li><%=docu.title%></li>
<%end%>
<% end %>
But of course it is bad as it is looping as many times as many documents I have. I have only one category and I want it to loop only once when list of links is printed.
With jekyll it was done like this (part of _includes nav.html from https://github.com/devo-ps/carte) :
{% for category in site.categories %}
<li><h2>{{ category | first }}</h2>
<ul>
{% for posts in category %}
{% for post in posts %}
<li class='{{ post.type }}'><a href='#{{ post.url }}'>{{ post.title }}</a></li>
{% endfor %}
{% endfor %}
</ul>
</li>
{% endfor %}
He somehow knows how many categories are there. I don't know how to port it to docpad

Answer 1

I think the best question is when after asking in you find the answer :)
So I found a "workaround" at least I think it is a workaround and not a solution. For me that is perfect:
I've added "categories" to docpad.coffee file
templateData:
site:
categories: ['Tutorials','General']
Now I will always update this array with categories that should be used in meta info of each markdown doc
My loop looks like this now....and works!!!
<% for category in #site.categories : %>
<li><h2><%- category %></h2>
<ul>
<%for docu in #getFilesAtPath("document-repository",[{filename: -1}]).findAll({category:category}).toJSON():%>
<li><%=docu.title%></li>
<% end %>
</ul>
</li>
<% end %>