removed non-orphans from root level of collections' hierarchy view

in order to avoid redundancies only orphans - i.e. collections without a
parent collection - should appear at the root level

the current implementation seems rather brute-force, i.e. inefficient
and inelegant - there might be a better way

NB: Circular references (e.g. foo contains bar, bar contains foo) result
in the respective collections not being displayed at all, rendering them
inaccessible (unless they have a separate relationship to an orphan).
This was deemed a data-entry issue, thus not relevant here.

app/views/collections/index.html.erb

@@ -9,5 +9,19 @@
       :format => :json)
 -%>
 <ul class="hybrid-treeview" data-url="<%= hcpath %>">
-  <%= render :partial => 'collection', :collection => @collections %>
+  <%-
+    # determine non-orphans (items with a parent)
+    children = []
+    @collections.each do |collection| # TODO: use #map along with #flatten?
+      if collection.subcollections.any?
+        children += collection.subcollections
+      end
+    end
+    children.uniq! # TODO: use Set?
+    # determine orphans (root-level items)
+    orphans = @collections.find_all do |collection|
+      not children.include? collection
+    end
+  -%>
+  <%= render :partial => 'collection', :collection => orphans %>
 </ul>