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, 8 insertions, 4 deletions

diff --git a/lib/sqlalchemy/ext/asyncio/engine.py b/lib/sqlalchemy/ext/asyncio/engine.py
index ab29438ed..a9e43a65f 100644
--- a/lib/sqlalchemy/ext/asyncio/engine.py
+++ b/lib/sqlalchemy/ext/asyncio/engine.py
@@ -113,7 +113,6 @@ class AsyncConnection(ProxyComparable, StartableContext, AsyncConnectable):
     def connection(self):
         """Not implemented for async; call
         :meth:`_asyncio.AsyncConnection.get_raw_connection`.
-
         """
         raise exc.InvalidRequestError(
             "AsyncConnection.connection accessor is not implemented as the "
@@ -125,9 +124,14 @@ class AsyncConnection(ProxyComparable, StartableContext, AsyncConnectable):
         """Return the pooled DBAPI-level connection in use by this
         :class:`_asyncio.AsyncConnection`.
 
-        This is typically the SQLAlchemy connection-pool proxied connection
-        which then has an attribute .connection that refers to the actual
-        DBAPI-level connection.
+        This is a SQLAlchemy connection-pool proxied connection
+        which then has the attribute
+        :attr:`_pool._ConnectionFairy.driver_connection` that refers to the
+        actual driver connection. Its
+        :attr:`_pool._ConnectionFairy.dbapi_connection` refers instead
+        to an :class:`_engine.AdaptedConnection` instance that
+        adapts the driver connection to the DBAPI protocol.
+
         """
         conn = self._sync_connection()