Postgres seems to need every column in the group by, not only the PK.

src/IDF/Tag.php

@@ -76,13 +76,14 @@ class IDF_Tag extends Pluf_Model
                             );
 
         $table = $this->_con->pfx.'idf_project_idf_tag_assoc';
+        $cols = implode(', ', array_keys($this->_a['cols']));
         $this->_a['views'] = array(
             'join_projects' =>
                 array(
                 'join' => 'LEFT JOIN '.$table
                         .' ON idf_tag_id=id',
                 'select' => $this->getSelect().',COUNT(idf_project_id) as project_count',
-                'group' => 'id',
+                'group' => $cols,
                 'props' => array('project_count' => 'project_count'),
                 ),
         );