Handle empty graph case on remote write back

Author: FlorentinD

changelog.md

@@ -9,7 +9,8 @@
 
 ## Bug fixes
 
-* Fixed a bug, where sessions could not be created for AuraDB instances of tier `business-critical`
+* Fixed a bug, where sessions could not be created for AuraDB instances of tier `business-critical`.
+* Fixed a bug, where sessions would fail on write-back if the Graph was empty.
 
 
 ## Improvements

graphdatascience/query_runner/session_query_runner.py

@@ -195,10 +195,19 @@ def _remote_write_back(
         def run_write_back() -> DataFrame:
             return write_protocol.run_write_back(self._db_query_runner, write_back_params, yields)
 
-        if self._resolve_show_progress(logging):
-            database_write_result = self._progress_logger.run_with_progress_logging(run_write_back, job_id, database)
-        else:
-            database_write_result = run_write_back()
+        try:
+            if self._resolve_show_progress(logging):
+                database_write_result = self._progress_logger.run_with_progress_logging(
+                    run_write_back, job_id, database
+                )
+            else:
+                database_write_result = run_write_back()
+        except Exception as e:
+            # catch the case nothing was needed to write-back (empty graph)
+            # once we have the Arrow Endpoints V2, we could catch by first checking the jobs summary
+            if "No entry with job id" in str(e) and gds_write_result.get("writeMillis", -1) == 0:
+                return gds_write_result
+            raise e
 
         write_millis = (time.time() - write_back_start) * 1000
         gds_write_result["writeMillis"] = write_millis

graphdatascience/tests/integration/test_remote_graph_ops.py

@@ -204,3 +204,19 @@ def test_remote_write_back_relationship_property_from_pathfinding_algo(
     )
 
     assert result["relationshipsWritten"] == 1
+
+
+@pytest.mark.cloud_architecture
+@pytest.mark.compatible_with(min_inclusive=ServerVersion(2, 7, 0))
+def test_empty_graph_write_back(
+    gds_with_cloud_setup: AuraGraphDataScience,
+) -> None:
+    G, result = gds_with_cloud_setup.graph.project(
+        GRAPH_NAME, "MATCH (n:MISSING) RETURN gds.graph.project.remote(n, null)"
+    )
+
+    assert G.node_count() == 0
+
+    result = gds_with_cloud_setup.wcc.write(G, writeProperty="wcc")
+
+    assert result["relationshipsWritten"] == 0