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/core.py

@@ -16,8 +16,10 @@ import sqlalchemy as sa
 import sqlalchemy.ext.asyncio as sa_asyncio
 import asyncpg
 
+from nominatim.db.sqlalchemy_schema import SearchTables
 from nominatim.config import Configuration
 from nominatim.api.status import get_status, StatusResult
+from nominatim.api.connection import SearchConnection
 
 class NominatimAPIAsync:
     """ API loader asynchornous version.
@@ -29,6 +31,7 @@ class NominatimAPIAsync:
 
         self._engine_lock = asyncio.Lock()
         self._engine: Optional[sa_asyncio.AsyncEngine] = None
+        self._tables: Optional[SearchTables] = None
 
 
     async def setup_database(self) -> None:
@@ -73,6 +76,7 @@ class NominatimAPIAsync:
                 # Make sure that all connections get the new settings
                 await self.close()
 
+            self._tables = SearchTables(sa.MetaData(), engine.name) # pylint: disable=no-member
             self._engine = engine
 
 
@@ -86,7 +90,7 @@ class NominatimAPIAsync:
 
 
     @contextlib.asynccontextmanager
-    async def begin(self) -> AsyncIterator[sa_asyncio.AsyncConnection]:
+    async def begin(self) -> AsyncIterator[SearchConnection]:
         """ Create a new connection with automatic transaction handling.
 
             This function may be used to get low-level access to the database.
@@ -97,9 +101,10 @@ class NominatimAPIAsync:
             await self.setup_database()
 
         assert self._engine is not None
+        assert self._tables is not None
 
         async with self._engine.begin() as conn:
-            yield conn
+            yield SearchConnection(conn, self._tables)
 
 
     async def status(self) -> StatusResult: