On Jan 26, 2011, at 5:16 PM, A.M. wrote:
> Hello,
>
> While working on a database test with nose, I dug into sqlalchemy 0.6.6 until
> I found these lines:
>
> class DDLElement(expression.Executable, expression.ClauseElement):
> """Base class for DDL expression constructs."""
>
> _execution_options = expression.Executable.\
> _execution_options.union({'autocommit':True})
>
> In my nose test against postgresql, I emit a bunch of DDL, perform the test
> and then roll back the whole shebang which conveniently makes it look like I
> didn't touch the database. Obviously, the emitted commits were getting in my
> way, so I wrote this:
>
> class NoCommitDDL(DDL):
> def __init__(self,*args,**kw):
> super(NoCommitDDL,self).__init__(*args,**kw)
> unfrozen = dict(self._execution_options)
> del unfrozen['autocommit']
> self._execution_options = frozendict(unfrozen)
>
> DDL = NoCommitDDL
>
> I still feel like I am missing something though. I understand that PostgreSQL
> is perhaps one of few databases to allow for transaction-aware DDL, but why
> is a commit emitted for the DDL for any database, when the database makes it
> implied anyway?
Several databases offer transactional DDL including Postgresql. SQLA doesn't
differentiate between these backends - it emits COMMIT after any statement that
it considers to be an "autocommit" statement. "autocommit" means, "commit this
statement after completion, if a transaction is not in progress".
>From this it follows that if you'd like to emit several DDL statements in a
>transaction, the usage is no different for DDL expressions than for any other
>kind of DML statement (i.e insert/update/delete). Use
>connection.begin()/transaction.commit() as documented at:
http://www.sqlalchemy.org/docs/core/connections.html#using-transactions
.
autocommit is described right after that:
http://www.sqlalchemy.org/docs/core/connections.html#understanding-autocommit
Also the public API for _execution_options is the execution_options()
generative call:
http://www.sqlalchemy.org/docs/core/expression_api.html?highlight=execution_options#sqlalchemy.sql.expression.Executable.execution_options
.
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