add SQLAlchemy table schema for Nominatim tables

Also defines an extended connection object that includes access to
the table definitions. Makes it easier to access the tables from
code that has been split off into separate modules.

nominatim/api/connection.py

@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2023 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Extended SQLAlchemy connection class that also includes access to the schema.
+"""
+from typing import Any, Mapping, Sequence, Union
+
+import sqlalchemy as sa
+from sqlalchemy.ext.asyncio import AsyncConnection
+
+from nominatim.db.sqlalchemy_schema import SearchTables
+
+class SearchConnection:
+    """ An extended SQLAlchemy connection class, that also contains
+        then table definitions. The underlying asynchronous SQLAlchemy
+        connection can be accessed with the 'connection' property.
+        The 't' property is the collection of Nominatim tables.
+    """
+
+    def __init__(self, conn: AsyncConnection,
+                 tables: SearchTables) -> None:
+        self.connection = conn
+        self.t = tables # pylint: disable=invalid-name
+
+
+    async def scalar(self, sql: sa.sql.base.Executable,
+                     params: Union[Mapping[str, Any], None] = None
+                    ) -> Any:
+        """ Execute a 'scalar()' query on the connection.
+        """
+        return await self.connection.scalar(sql, params)
+
+
+    async def execute(self, sql: sa.sql.base.Executable,
+                      params: Union[Mapping[str, Any], Sequence[Mapping[str, Any]], None] = None
+                     ) -> sa.engine.Result:
+        """ Execute a 'execute()' query on the connection.
+        """
+        return await self.connection.execute(sql, params)