"Mike Bayer" <mike...@zzzcomputing.com> writes:
> On Sat, Jul 24, 2021, at 5:49 AM, Lele Gaifax wrote:
>> Hi all,
>>
>> I need some clarification on the following RemovedIn20Warning
>>
>> "Pet" object is being merged into a Session along the backref cascade path
>> for relationship "Person.pets"; in SQLAlchemy 2.0, this reverse cascade
>> will
>> not take place...
>>
>> Does it really means that in SA 2.0 the pattern
>>
>> new_entity = Entity(attached_to_object_id=1)
>>
>> won't work at all, or instead that the "other end" one-to-many relationship
>> list won't be automatically updated? If the latter, is there any way to say
>> "ok, I don't care about the reverse cascade updates in this particular case,
>> as no further accesses to the one-to-many rels on the other side will
>> happen"?
>
> so you want to use the "new" mode of operation which is given if you either
> set future=True on Session , or on specific relationships you set
> cascade_backrefs=False. Then you want to make sure you add() to the Session
> all the objects that you want persisted.
Oh, thank you: then I simply got the meaning of the warning wrong, ie I
assumed it was related to adding the new object to the "other side ref
collection" (that is, Person.pets), not about registering it as pending within
the session.
>
>>
>> # obtain "owner" ID from the request
>> owners = {key: value for key in request.matchdict if key.endswith('_id')}
>> pet = Pet(**owners)
>> session.add(pet)
>> # apply form's data
>> pet.edit(request.POST.items())
>> session.commit()
>>
>> This triggers the mentioned SA 1.4 warning when executed with
>> SQLALCHEMY_WARN_20=1, as the attached snippet exhibits.
>
> OK so above, you get the warning on `Pet(**owners)`, but then you are doing
> session.add(pet), so your code is already correct and you can turn off
> backref cascades for that.
Great, will try that out!
>
>
>>
>> I tried several variants, such as using "back_populates" instead of the "old"
>> backref...
>>
>> So, am I missing something and there is some recommended alternative, or
>> should I dig into the rabbit's hole?
>
> The warning itself refers to the "cascade_backrefs" flag as well as an FAQ
> entry on this at
> https://docs.sqlalchemy.org/en/14/errors.html#object-is-being-merged-into-a-session-along-the-backref-cascade
> , as well as the fact that you can turn off "cascade_backrefs" across the
> board with
> Session(future=True) if you don't want to have to dig into individual
> relationships.
>
> is there a reason you were trying other settings (for example back_populates
> vs. backref, these two syntaxes are equivalent and aren't related here) and
> not the one that's documented? I'm just wondering if you missed seeing that.
No particular reason, except my misunderstanding and overlooking that chapter
in errors.html (I'm quite sure I visited /one/ of the two shorturls in the
message, evidently the second, the one pointing to "some function in SA 2.0
will no longer to something", and that didn't turn the light on).
Wrt back_populates, you know, I'm a long running SA user, and still use
backref syntax: I wasn't sure they are just equivalent alternatives.
Thanks a lot for clearing this out!
ciao, lele.
--
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
l...@metapensiero.it | -- Fortunato Depero, 1929.
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