fix core api with plain old select

src/questdb_connect/compilers.py

@@ -72,7 +72,7 @@ def group_by_clause(self, select, **kw):
         text = ""
 
         # Add SAMPLE BY first if present
-        if select._sample_by_clause is not None:
+        if _has_sample_by(select):
             text += " " + self.process(select._sample_by_clause, **kw)
 
         # Use parent's GROUP BY implementation
@@ -88,7 +88,7 @@ def visit_select(self, select, **kw):
         # If we have SAMPLE BY but no GROUP BY,
         # add a dummy GROUP BY clause to trigger the rendering
         if (
-                select._sample_by_clause is not None
+                _has_sample_by(select)
                 and not select._group_by_clauses
         ):
             select = select._clone()
@@ -103,3 +103,6 @@ def _is_safe_for_fast_insert_values_helper(self):
     def visit_textclause(self, textclause, add_to_result_map=None, **kw):
         textclause.text = remove_public_schema(textclause.text)
         return super().visit_textclause(textclause, add_to_result_map, **kw)
+
+def _has_sample_by(select):
+    return hasattr(select, '_sample_by_clause') and select._sample_by_clause is not None

tests/test_dialect.py

@@ -516,6 +516,40 @@ def test_sample_by_from_to(test_engine, test_model):
         if session:
             session.close()
 
+def test_plain_select_core_api(test_engine, test_model):
+    """
+    Test plain select with core API. Plain select means select implementation from sqlalchemy.sql.selectable,
+    not from questdb_connect.
+    """
+
+    session = Session(test_engine)
+    try:
+        num_rows = 3
+        models = [
+            test_model(
+                col_int=idx,
+                col_ts=datetime.datetime(2023, 4, 12, 0, 0, 0) + datetime.timedelta(hours=idx),
+            ) for idx in range(num_rows)
+        ]
+        session.bulk_save_objects(models)
+        session.commit()
+
+        metadata = sqla.MetaData()
+        table = sqla.Table(ALL_TYPES_TABLE_NAME, metadata, autoload_with=test_engine)
+        wait_until_table_is_ready(test_engine, ALL_TYPES_TABLE_NAME, len(models))
+
+        with test_engine.connect() as conn:
+            query = (
+                # important: use sqla.select, not questdb_connect.select!
+                sqla.select(table.c.col_ts, table.c.col_int)
+            )
+            result = conn.execute(query)
+            rows = result.fetchall()
+            assert len(rows) == 3
+    finally:
+        if session:
+            session.close()
+
 def test_sample_by_options(test_engine, test_model):
     """Test SAMPLE BY with ALIGN TO and FILL options."""
     base_ts = datetime.datetime(2023, 4, 12, 0, 0, 0)