Implement connection binding for AsyncSession

Implemented "connection-binding" for :class:`.AsyncSession`, the ability to
pass an :class:`.AsyncConnection` to create an :class:`.AsyncSession`.
Previously, this use case was not implemented and would use the associated
engine when the connection were passed.  This fixes the issue where the
"join a session to an external transaction" use case would not work
correctly for the :class:`.AsyncSession`.  Additionally, added methods
:meth:`.AsyncConnection.in_transaction`,
:meth:`.AsyncConnection.in_nested_transaction`,
:meth:`.AsyncConnection.get_transaction`.

The :class:`.AsyncEngine`, :class:`.AsyncConnection` and
:class:`.AsyncTransaction` objects may be compared using Python ``==`` or
``!=``, which will compare the two given objects based on the "sync" object
they are proxying towards. This is useful as there are cases particularly
for :class:`.AsyncTransaction` where multiple instances of
:class:`.AsyncTransaction` can be proxying towards the same sync
:class:`_engine.Transaction`, and are actually equivalent.   The
:meth:`.AsyncConnection.get_transaction` method will currently return a new
proxying :class:`.AsyncTransaction` each time as the
:class:`.AsyncTransaction` is not otherwise statefully associated with its
originating :class:`.AsyncConnection`.

Fixes: #5811
Change-Id: I5a3a6b2f088541eee7b0e0f393510e61bc9f986b

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/sqlalchemy/ext/asyncio/session.py b/lib/sqlalchemy/ext/asyncio/session.py
index bac2aa44b..9a8284e64 100644
--- a/lib/sqlalchemy/ext/asyncio/session.py
+++ b/lib/sqlalchemy/ext/asyncio/session.py
@@ -75,12 +75,13 @@ class AsyncSession:
         kw["future"] = True
         if bind:
             self.bind = engine
-            bind = engine._get_sync_engine(bind)
+            bind = engine._get_sync_engine_or_connection(bind)
 
         if binds:
             self.binds = binds
             binds = {
-                key: engine._get_sync_engine(b) for key, b in binds.items()
+                key: engine._get_sync_engine_or_connection(b)
+                for key, b in binds.items()
             }
 
         self.sync_session = self._proxied = Session(