Surface driver connection object when using a proxied dialect

Improve the interface used by adapted drivers, like the asyncio ones,
to access the actual connection object returned by the driver.

The :class:`_engine._ConnectionRecord` and
:class:`_engine._ConnectionFairy` now have two new attributes:

* ``dbapi_connection`` always represents a DBAPI compatible
object.  For pep-249 drivers, this is the DBAPI connection as it always
has been, previously accessed under the ``.connection`` attribute.
For asyncio drivers that SQLAlchemy adapts into a pep-249 interface,
the returned object will normally be a SQLAlchemy adaption object
called :class:`_engine.AdaptedConnection`.
* ``driver_connection`` always represents the actual connection object
maintained by the third party pep-249 DBAPI or async driver in use.
For standard pep-249 DBAPIs, this will always be the same object
as that of the ``dbapi_connection``.  For an asyncio driver, it will be
the underlying asyncio-only connection object.

The ``.connection`` attribute remains available and is now a legacy alias
of ``.dbapi_connection``.

Fixes: #6832
Change-Id: Ib72f97deefca96dce4e61e7c38ba430068d6a82e

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 8bd8a121b..eff28e340 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -646,7 +646,7 @@ class DefaultDialect(interfaces.Dialect):
                         % (", ".join(name for name, obj in trans_objs))
                     )
 
-        dbapi_connection = connection.connection.connection
+        dbapi_connection = connection.connection.dbapi_connection
         for name, characteristic, value in characteristic_values:
             characteristic.set_characteristic(self, dbapi_connection, value)
         connection.connection._connection_record.finalize_callback.append(
@@ -779,6 +779,9 @@ class DefaultDialect(interfaces.Dialect):
                 name = unicode(name)  # noqa
         return name
 
+    def get_driver_connection(self, connection):
+        return connection
+
 
 class _RendersLiteral(object):
     def literal_processor(self, dialect):