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.
>
> # 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.
>
> I tried several variants, such as using "back_populates" instead of the "old"
> backref, but the only pattern that removed the warning has been
>
> owner = session.get(Person, request.matchdict['person_id'])
> pet = Pet()
> owner.pets.append(pet)
>
> where I surely can get, if needed, but would require implementing more
> per-entity specific knowledge (possibly using introspection) into the generic
> CRUD machinery, to "convert" from the "person_id" to the name of the "other
> side" relationship attribute, in several dozens of places.
>
> Another approach could be configuring all the "one-to-many" relationships as
> "viewonly", but that again would require more analysis on the code paths...
>
> 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.
>
> Thanks in advance for any hint,
> ciao, lele.
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
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> --
> 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.
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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>
> *Attachments:*
> * test.py
--
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http://www.sqlalchemy.org/
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