Fix union SQL and selection previews

tests/test_graph_and_sql.py

@@ -288,6 +288,27 @@ class DataflowGraphAndSqlTests(unittest.TestCase):
         result = execute_preview(graph, row_limit=100)
 
         self.assertEqual([{"id": 1}, {"id": 2}, {"id": 3}], result.rows)
+        rendered, diagnostics = render_sql(graph)
+        self.assertEqual([], diagnostics)
+        self.assertIn("UNION BY NAME", rendered)
+        roundtrip, normalized, diagnostics = compile_sql(
+            rendered,
+            source_nodes=[first, second],
+        )
+        self.assertEqual(rendered, normalized)
+        self.assertEqual([], diagnostics)
+        self.assertEqual(
+            ["source.inline", "source.inline", "combine.union", "output"],
+            [node.type for node in roundtrip.nodes],
+        )
+        self.assertEqual(result.rows, execute_preview(roundtrip, row_limit=100).rows)
+
+    def test_legacy_composite_edge_ids_remain_readable(self) -> None:
+        edge_id = f"edge-{'a' * 100}-{'b' * 100}"
+
+        edge = GraphEdge(id=edge_id, source="source", target="output")
+
+        self.assertEqual(edge_id, edge.id)
 
     def test_connector_source_uses_bounded_resolver_and_records_lineage(self) -> None:
         source = GraphNode(