MBS-10076: Friendlier error when deleting attribute that has children

Right now this just ISEs - an actual error explaining the issue
seems friendlier. This is just an admin page, but we might as well.

Author: reosarevok

lib/MusicBrainz/Server/Controller/Admin/Attributes.pm

@@ -1,6 +1,8 @@
 package MusicBrainz::Server::Controller::Admin::Attributes;
 use Moose;
 
+use MusicBrainz::Server::Translation qw( l ln );
+
 no if $] >= 5.018, warnings => "experimental::smartmatch";
 
 BEGIN { extends 'MusicBrainz::Server::Controller' };
@@ -125,9 +127,18 @@ sub delete : Chained('attribute_base') Args(1) RequireAuth(account_admin) Secure
 
         $c->detach;
     }
+
+    if ($c->model($model)->has_children($id)) {
+        my $error_message = l('You cannot remove the attribute “{name}” because it is the parent of other attributes.', { name => $attr->name });
+
+        $c->stash(
+            current_view => 'Node',
+            component_path => 'admin/attributes/CannotRemoveAttribute',
+            component_props => {message => $error_message}
+        );
+        
         $c->detach;
     }
-
     if ($c->form_posted_and_valid($form)) {
         $c->model('MB')->with_transaction(sub {
             $c->model($model)->delete($id);

lib/MusicBrainz/Server/Data/Role/Attribute.pm

@@ -19,6 +19,13 @@ sub _column_mapping {
     };
 }
 
+sub has_children {
+    my ($self, $id) = @_;
+    return $self->sql->select_single_value(
+        'SELECT 1 FROM ' . $self->_table . ' WHERE parent = ? LIMIT 1',
+        $id);
+}
+
 no Moose;
 1;