Update docs

docs/transactions.rst

@@ -7,12 +7,29 @@ Transactions
 Tortoise ORM provides a simple way to manage transactions. You can use the
 ``atomic()`` decorator or ``in_transaction()`` context manager.
 
-``atomic()`` and ``in_transaction()`` can be nested, and the outermost block will
-be the one that actually commits the transaciton. Tortoise ORM doesn't support savepoints yet.
+``atomic()`` and ``in_transaction()`` can be nested. The inner blocks will create transaction savepoints,
+and if an exception is raised and then caught outside of a nested block, the transaction will be rolled back
+to the state before the block was entered. The outermost block will be the one that actually commits the transaction.
+The savepoints are supported for Postgres, SQLite, MySQL and SQLite.
 
-In most cases ``asyncio.gather`` or similar ways to spin up concurrent tasks can be used safely
-when querying the database or using transactions. Tortoise ORM will ensure that for the duration
-of a query, the database connection is used exclusively by the task that initiated the query.
+  .. code-block:: python3
+
+    # this block will commit changes on exit
+    async with in_transaction():
+        await MyModel.create(name='foo')
+        try:
+            # this block will create a savepoint
+            async with in_transaction():
+                await MyModel.create(name='bar')
+                # this will rollback to the savepoint, meaning that
+                # the 'bar' record will not be created
+                raise Exception()
+        except Exception:
+            pass
+
+When using ``asyncio.gather`` or similar ways to spin up concurrent tasks in a transaction block,
+avoid having nested transaction blocks in the concurrent tasks. Transactions are stateful and nested
+blocks are expected to run sequentially, not concurrently.
 
 
 .. automodule:: tortoise.transactions

tortoise/backends/mysql/client.py

@@ -17,6 +17,7 @@
     import asyncmy as mysql
     from asyncmy import errors
     from asyncmy.charset import charset_by_name
+    from asyncmy.constants import COMMAND
 except ImportError:
     import aiomysql as mysql
     from pymysql.charset import charset_by_name

tortoise/backends/odbc/client.py

@@ -205,13 +205,10 @@ async def rollback(self) -> None:
         self._connection._conn.autocommit = True
 
     async def savepoint(self) -> None:
-        # TODO
         pass
 
     async def savepoint_rollback(self) -> None:
-        # TODO
         pass
 
     async def release_savepoint(self) -> None:
-        # TODO
         pass