Check for empty aggregation result

Author: FlorentinD

changelog.md

@@ -22,5 +22,7 @@
 
 ## Improvements
 
+* Improved the error message if `gds.graph.project.cypher` produces an empty graph.
+
 
 ## Other changes

graphdatascience/graph/graph_cypher_runner.py

@@ -1,7 +1,11 @@
+from __future__ import annotations
+
 import re
 from itertools import chain, zip_longest
 from typing import Any, Optional
 
+from pandas import Series
+
 from ..caller_base import CallerBase
 from ..query_runner.query_runner import QueryRunner
 from ..server_version.server_version import ServerVersion
@@ -41,7 +45,10 @@ def project(
 
         GraphCypherRunner._verify_query_ends_with_return_clause(self._namespace, query)
 
-        result = self._query_runner.run_cypher(query, params, database, False).squeeze()
+        result: Optional[Series[Any]] = self._query_runner.run_cypher(query, params, database, False).squeeze()
+
+        if result is None or result.empty:
+            raise ValueError("Projected graph cannot be empty.")
 
         try:
             graph_name = str(result["graphName"])

graphdatascience/tests/integration/test_graph_ops.py

@@ -94,6 +94,12 @@ def test_cypher_projection(gds: GraphDataScience) -> None:
     assert result["exists"]
 
 
+@pytest.mark.filterwarnings("ignore: One of the labels in your query is not available in the database")
+def test_cypher_projection_empty_graph(gds: GraphDataScience) -> None:
+    with pytest.raises(ValueError, match="Projected graph cannot be empty"):
+        gds.graph.cypher.project("MATCH (n:MISSING_LABEL) RETURN gds.graph.project('some-graph', n, null)")
+
+
 def test_beta_project_subgraph(runner: QueryRunner, gds: GraphDataScience) -> None:
     from_G, _ = gds.graph.project(GRAPH_NAME, {"Node": {"properties": "x"}}, "*")