Various optimizations:

o Add model_cache::get() which caches models avoiding duplicate lookups
o Stop using ORM relationships for Item_Model::owner so that we can use caching
o For Item_Model::xxx_edit fields, don't make them editable for guests
o Other minor stuff.

These optimizations reduce the number of queries for a 9-photos page from ~200
to ~45.  Still way too many!

core/helpers/model_cache.php

@@ -0,0 +1,35 @@
+<?php defined("SYSPATH") or die("No direct script access.");
+/**
+ * Gallery - a web based photo album viewer and editor
+ * Copyright (C) 2000-2008 Bharat Mediratta
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or (at
+ * your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+class model_cache_Core {
+  private static $cache;
+
+  function get($model_name,  $id, $field_name="id") {
+    if (TEST_MODE || empty(self::$cache->$model_name->$field_name->$id)) {
+      $model = ORM::factory($model_name)->where($field_name, $id)->find();
+      if (!$model->loaded) {
+        throw new Exception("@todo MISSING_MODEL $model_name:$id");
+      }
+      self::$cache->$model_name->$field_name->$id = $model;
+    }
+
+
+    return self::$cache->$model_name->$field_name->$id;
+  }
+}