Fixed #9206 -- Clarified documentation of transaction handling in raw SQL, and error recovery for Postgres. Thanks to Richard Davies for the suggestion and draft text.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10655 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/db/models.txt

@@ -752,37 +752,8 @@ Executing custom SQL
 --------------------
 
 Another common pattern is writing custom SQL statements in model methods and
-module-level methods. The object :class:`django.db.connection
-<django.db.backends.DatabaseWrapper>` represents the current database
-connection. To use it, call :meth:`connection.cursor()
-<django.db.backends.DatabaseWrapper.cursor>` to get a cursor object. Then, call
-``cursor.execute(sql, [params])`` to execute the SQL and
-:meth:`cursor.fetchone() <django.db.backends.CursorWrapper.fetchone>` or
-:meth:`cursor.fetchall() <django.db.backends.CursorWrapper.fetchall>` to return
-the resulting rows. For example::
-
-    def my_custom_sql(self):
-        from django.db import connection
-        cursor = connection.cursor()
-        cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
-        row = cursor.fetchone()
-        return row
-
-:class:`connection <django.db.backends.DatabaseWrapper>` and :class:`cursor
-<django.db.backends.CursorWrapper>` mostly implement the standard Python
-DB-API -- see :pep:`249` -- with the addition of Django's :ref:`transaction
-handling <topics-db-transactions>`. If you're not familiar with the Python
-DB-API, note that the SQL statement in :meth:`cursor.execute()
-<django.db.backends.CursorWrapper.execute>` uses placeholders, ``"%s"``, rather
-than adding parameters directly within the SQL. If you use this technique, the
-underlying database library will automatically add quotes and escaping to your
-parameter(s) as necessary. (Also note that Django expects the ``"%s"``
-placeholder, *not* the ``"?"`` placeholder, which is used by the SQLite Python
-bindings. This is for the sake of consistency and sanity.)
-
-A final note: If all you want to do is a custom ``WHERE`` clause, you can use
-the :meth:`~QuerySet.extra` lookup method, which lets you add custom SQL to a
-query.
+module-level methods. For more details on using raw SQL, see the documentation
+on :ref:`using raw SQL<topics-db-sql>`.
 
 .. _model-inheritance:
 

docs/topics/db/sql.txt

@@ -5,16 +5,28 @@ Performing raw SQL queries
 
 Feel free to write custom SQL statements in custom model methods and
 module-level methods. The object ``django.db.connection`` represents the
-current database connection. To use it, call ``connection.cursor()`` to get a
-cursor object. Then, call ``cursor.execute(sql, [params])`` to execute the SQL
-and ``cursor.fetchone()`` or ``cursor.fetchall()`` to return the resulting
-rows. Example::
+current database connection, and ``django.db.transaction`` represents the
+current database transaction. To use the database connection, call
+``connection.cursor()`` to get a cursor object. Then, call
+``cursor.execute(sql, [params])`` to execute the SQL and ``cursor.fetchone()``
+or ``cursor.fetchall()`` to return the resulting rows. After performing a data
+changing operation, you should then call
+``transaction.commit_unless_managed()`` to ensure your changes are committed
+to the database. If your query is purely a data retrieval operation, no commit
+is required. For example::
 
     def my_custom_sql(self):
-        from django.db import connection
+        from django.db import connection, transaction
         cursor = connection.cursor()
+
+        # Data modifying operation - commit required
+        cursor.execute("UPDATE bar SET foo = 1 WHERE baz = %s", [self.baz])
+        transaction.commit_unless_managed()
+
+        # Data retrieval operation - no commit required
         cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
         row = cursor.fetchone()
+
         return row
 
 ``connection`` and ``cursor`` mostly implement the standard `Python DB-API`_

docs/topics/db/transactions.txt

@@ -10,10 +10,10 @@ if you're using a database that supports transactions.
 Django's default transaction behavior
 =====================================
 
-Django's default behavior is to commit automatically when any built-in,
-data-altering model function is called. For example, if you call
-``model.save()`` or ``model.delete()``, the change will be committed
-immediately.
+Django's default behavior is to run with an open transaction which it
+commits automatically when any built-in, data-altering model function is
+called. For example, if you call ``model.save()`` or ``model.delete()``, the
+change will be committed immediately.
 
 This is much like the auto-commit setting for most databases. As soon as you
 perform an action that needs to write to the database, Django produces the
@@ -165,3 +165,29 @@ they're called. If your MySQL setup *does* support transactions, Django will
 handle transactions as explained in this document.
 
 .. _information on MySQL transactions: http://dev.mysql.com/doc/refman/5.0/en/sql-syntax-transactions.html
+
+Transactions and savepoints in PostgreSQL 8
+===========================================
+
+When a call to a PostgreSQL 8 cursor raises an exception, all subsequent SQL
+in the same transaction fails with the error "current transaction is aborted,
+queries ignored until end of transaction block". Whilst simple use of save()
+is unlikely to raise an exception in PostgreSQL, there are many more advanced
+usage patterns which might: for example, saving objects with unique fields,
+saving using the force_insert/force_update flag, or invoking custom SQL.
+
+In any of these cases, you can wrap the command which may throw
+IntegrityError inside savepoints, which will then allow subsequent commands
+to proceed. Example::
+
+    try:
+      sid = transaction.savepoint()
+      x.save()
+      transaction.savepoint_commit(sid)
+    except IntegrityError:
+      transaction.savepoint_rollback(sid)
+      raise
+
+Savepoints are not implemented in PostgreSQL 7. If you experience an
+IntegrityError when using PostgreSQL 7, you will need to rollback to the
+start of the transaction.