Forms: used templates should not be deleted

library/Director/Objects/IcingaObject.php

@@ -530,6 +530,19 @@ abstract class IcingaObject extends DbObject implements IcingaConfigRenderer
         return $this;
     }
 
+    public function countDirectDescendants()
+    {
+        $db = $this->getDb();
+        $table = $this->getTableName();
+        $type = strtolower($this->getType());
+        $query = $db->select()->from(
+            array('oi' => $table . '_inheritance'),
+            array('cnt' => 'COUNT(*)')
+        )->where('oi.parent_' . $type . '_id = ?', (int) $this->id);
+
+        return $db->fetchOne($query);
+    }
+
     protected function resolve($what)
     {
         if ($this->hasResolveCached($what)) {

library/Director/Web/Form/DirectorObjectForm.php

@@ -684,6 +684,8 @@ abstract class DirectorObjectForm extends QuickForm
 
     protected function addDeleteButton($label = null)
     {
+        $object = $this->object;
+
         if ($label === null) {
             $label = $this->translate('Delete');
         }
@@ -694,6 +696,20 @@ abstract class DirectorObjectForm extends QuickForm
             //->removeDecorator('Label');
 
         $this->deleteButtonName = $el->getName();
+
+        if ($object instanceof IcingaObject && $object->isTemplate()) {
+            if ($cnt = $object->countDirectDescendants()) {
+                $el->setAttrib('disabled', 'disabled');
+                $el->setAttrib(
+                    'title',
+                    sprintf(
+                        $this->translate('This template is still in use by %d other objects'),
+                        $cnt
+                    )
+                );
+            }
+        }
+
         $this->addElement($el);
 
         return $this;

public/css/module.less

@@ -52,6 +52,12 @@ form input[type=submit] {
   .button();
   border-width: 1px;
   margin-top: 0.5em;
+
+  &:disabled {
+    border-color: @gray-light;
+    background-color: @gray-light;
+    color: #fff;
+  }
 }
 
 form input[type=submit]:first-of-type {