Handle missing root communities in dynamic selection

packages/graphrag/graphrag/query/context_builder/dynamic_community_selection.py

@@ -67,8 +67,8 @@ def __init__(
             if community.short_id in self.reports:
                 self.levels[community.level].append(community.short_id)
 
-        # start from root communities (level 0)
-        self.starting_communities = self.levels["0"]
+        # start from root communities (level 0), if any
+        self.starting_communities = self.levels.get("0", [])
 
     async def select(self, query: str) -> tuple[list[CommunityReport], dict[str, Any]]:
         """

tests/unit/query/context_builder/dynamic_community_selection.py

@@ -5,6 +5,7 @@
 
 from unittest.mock import MagicMock
 
+import pytest
 from graphrag.data_model.community import Community
 from graphrag.data_model.community_report import CommunityReport
 from graphrag.query.context_builder.dynamic_community_selection import (
@@ -203,3 +204,49 @@ def test_dynamic_community_selection_handles_str_children():
         assert child_id in selector.reports, (
             f"Child {child} (as '{child_id}') should be found in reports"
         )
+
+
+@pytest.mark.asyncio
+async def test_dynamic_community_selection_handles_missing_level_zero():
+    """Test that DynamicCommunitySelection does not require a level 0 community."""
+    communities = [
+        Community(
+            id="comm-1",
+            short_id="1",
+            title="Child Community 1",
+            level="1",
+            parent="0",
+            children=[],
+        ),
+    ]
+    reports = [
+        CommunityReport(
+            id="report-1",
+            short_id="1",
+            title="Report 1",
+            community_id="1",
+            summary="Child 1 summary",
+            full_content="Child 1 full content",
+            rank=1.0,
+        ),
+    ]
+
+    model = create_mock_model()
+    tokenizer = create_mock_tokenizer()
+
+    selector = DynamicCommunitySelection(
+        community_reports=reports,
+        communities=communities,
+        model=model,
+        tokenizer=tokenizer,
+        threshold=1,
+        keep_parent=False,
+        max_level=2,
+    )
+
+    assert selector.starting_communities == []
+    selected_reports, llm_info = await selector.select("query")
+
+    assert selected_reports == []
+    assert llm_info["llm_calls"] == 0
+    model.assert_not_called()