diff --git a/lektor_groupby/groupby.py b/lektor_groupby/groupby.py
index 372ccd8..6dfd888 100644
--- a/lektor_groupby/groupby.py
+++ b/lektor_groupby/groupby.py
@@ -22,6 +22,11 @@ class GroupBy:
         self._watcher = []  # type: List[Watcher]
         self._results = []  # type: List[GroupBySource]
         self.resolver = resolver
+        self.didBuild = False
+
+    @property
+    def isNew(self) -> bool:
+        return not self.didBuild
 
     def add_watcher(self, key: str, config: 'AnyConfig') -> Watcher:
         ''' Init Config and add to watch list. '''
@@ -60,6 +65,7 @@ class GroupBy:
 
     def make_once(self, builder: 'Builder') -> None:
         ''' Perform groupby, iter over sources with watcher callback. '''
+        self.didBuild = True
         if self._watcher:
             self.resolver.reset()
             for w in self._watcher:
diff --git a/lektor_groupby/plugin.py b/lektor_groupby/plugin.py
index 659e6ce..9fd8c95 100644
--- a/lektor_groupby/plugin.py
+++ b/lektor_groupby/plugin.py
@@ -1,4 +1,4 @@
-from lektor.db import Page  # isinstance
+from lektor.assets import File, Directory  # isinstance
 from lektor.pluginsystem import Plugin  # subclass
 from typing import TYPE_CHECKING, Iterator, Any
 from .backref import GroupByRef, VGroups
@@ -27,8 +27,11 @@ class GroupByPlugin(Plugin):
     ) -> None:
         # before-build may be called before before-build-all (issue #1017)
         # make sure it is always evaluated first
-        if isinstance(source, Page):
-            self._init_once(builder)
+        if isinstance(source, (File, Directory)):
+            return
+        groupby = self._init_once(builder)
+        if groupby.isNew and isinstance(source, GroupBySource):
+            self.has_changes = True
 
     def on_after_build(self, build_state: 'BuildState', **extra: Any) -> None:
         if build_state.updated_artifacts: