Re: Atomic block termination question

Carl

Thank you very much. Great overview. Confidence boosting.

Mike

On 26/06/2015 2:57 AM, Carl Meyer wrote:
> Hi Mike,
>
> On 06/25/2015 01:53 AM, Mike Dewhirst wrote:
>> On 25/06/2015 2:34 AM, Carl Meyer wrote:
>>> On 06/24/2015 02:16 AM, Mike Dewhirst wrote:
>>>> On 24/06/2015 4:43 PM, Mike Dewhirst wrote:
>>>>> When saving a model I'm getting a TransactionManagementError - You
>>>>> can't
>>>>> execute queries until the end of the 'atomic' block
>>>>>
>>>>> Ticket #21540 seems fairly explicit at least where Postgres is
>>>>> concerned. TransactionManagementError prevents what I want to do and
>>>>> I'm
>>>>> not a nuclear expert.
>>>>>
>>>>> How do I terminate the save() method code in that atomic block and then
>>>>> immediately execute my queries?
>>>
>>> I'm afraid this description of what you're trying to do is too vague to
>>> be useful. Maybe some sample code would help?
>>>
>>> TransactionManagementError is a symptom, not a cause. It means that a
>>> database error occurred inside a transaction, which leaves the
>>> transaction in an unpredictable state, so Postgres wants you to roll
>>> back the transaction (or roll back to a savepoint prior to the error)
>>> before issuing any more database queries.
>>
>> Ok. I thought from reading the ticket that I was trying to do something
>> illegal in Postgres - that is issuing a query within a transaction which
>> needed to be finalised or rolled back. I took it as a symptom or signal
>> and think I understand that Postgres is somewhat more rigorous in this
>> regard than MySQL.
>
> Yes, that's right. (Well, the transaction needs to be rolled back. Once
> there's been an error, there is no other "finalizing" possible besides a
> rollback.) My point was that the transaction gets into this state
> because some other query causes an error, so your efforts should first
> focus on figuring out what that error was and making it not happen, if
> possible.
>
>>>
>>> Possible solutions include:
>>>
>>> a) Figuring out why there was a database error, and fixing it so it
>>> doesn't occur.
>>
>> I separated out all the pre and post-save stuff without the offending
>> queries and put them into ...
>>
>> def save(self, *args, **kwargs):
>> self._pre_save() # nothing tricky here
>> super(Substance, self).save(*args, **kwargs)
>> self._post_save() # nothing tricky here
>>
>> ... which stopped the TransactionManagementError and everything worked
>> on existing substances which already had the necessary 1:1 relations AND
>> it kinda "worked" when I [saved as new] except obviously the 1:1
>> relations were not created.
>>
>> ... then did a _post_post_save() with the offending queries ...
>>
>> def _post_post_save(self):
>> if self.physical_state == SOLID:
>> Solid.objects.get_or_create(substance=self)
>> elif self.physical_state == LIQUID:
>> Liquid.objects.get_or_create(substance=self)
>> elif self.physical_state == GAS:
>> Gas.objects.get_or_create(substance=self)
>> elif self.physical_state == AEROSOL:
>> Aerosol.objects.get_or_create(substance=self)
>> Aquatic.objects.get_or_create(substance=self)
>> Tox.objects.get_or_create(substance=self)
>> Terrestrial.objects.get_or_create(substance=self)
>> if self.terrestrial:
>> # We can't do this in terrestrial.save() and it needs
>> # to be recomputed on every save
>> self.terrestrial.set_soil_m_factor()
>> self.terrestrial.set_vertebrate_m_factor()
>> self.terrestrial.set_invertebrate_m_factor()
>>
>> ... which as I said is now called from substance.clean(). I realise
>> clean() is called before save() but that's all I can think of at the
>> moment. Those m_factors are unlikely to change once the concentration
>> values (EC50, LD50 etc) upon which they are based are set.
>
> I don't understand why you want to call this from clean() instead of
> from save(), but it should work OK as long as you wrap it in an `if
> self.pk:` so it doesn't try to run on an unsaved object. Of course that
> means it will never run at all when saving a new object.
>
>>> b) Wrapping the code that might cause a database error in its own
>>> `transaction.atomic` block, so on error that bit of code is rolled back
>>> and later queries within the same transaction can go forward.
>>
>> That sounds like nuclear physics to me. I could probably follow a recipe
>> but might have trouble figuring out when to use it.
>
> Here's the general recipe. This code is problematic:
>
> @transaction.atomic
> def do_something():
> do_the_first_thing()
> try:
> do_a_thing_that_might_cause_a_database_error()
> except DatabaseError:
> pass
> do_another_thing()
>
>
> If `do_a_thing_that_might_cause_a_database_error()` does in fact cause a
> database error, then `do_another_thing()` will fail with
> `TransactionManagementError` (assuming it tries to use the database)
> because it's trying to continue with a transaction that's in an error state.
>
> Assuming you can't prevent the possibility of the database error, here
> is how you would solve the problem with the above code:
>
> @transaction.atomic
> def do_something():
> do_the_first_thing()
> try:
> with transaction.atomic():
> do_a_thing_that_might_cause_a_database_error()
> except DatabaseError:
> pass
> do_another_thing()
>
> By wrapping `do_a_thing_that_might_cause_a_database_error()` in an inner
> atomic block (that is, a savepoint), and catching the DatabaseError
> outside that atomic block, you allow the DatabaseError to first cause a
> rollback of that inner atomic block, putting the transaction back into a
> usable state.
>
> This is covered in the docs. See the warning in this section:
> https://docs.djangoproject.com/en/1.8/topics/db/transactions/#controlling-transactions-explicitly
>
>
> Carl
>

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