Re: [sqlalchemy] Possible regression?
Awesome! Thanks! -Lyla On Mon, Apr 29, 2019 at 1:55 PM Mike Bayer wrote: > On Mon, Apr 29, 2019 at 1:37 PM Lyla Fischer wrote: > > > > Ah. I think that I have been thinking of a session as a cache instead of > a transaction, which is incorrect. (It's similar because you have to worry > about persistence in the face of failures, but not-similar, because you're > not worried about communication with other processes. That is, a commit is > not "expose this data to other cores/machines". A commit is a checkpoint to > rollback to.) I was thinking that adding something to the session was > taking something from a local and putting it into a cache, and then > committing it was taking it from a cache and putting it into the database. > Totally wrong abstraction, sorry. > > OK, this is more thinking than I usually do about this on a daily > basis and I don't think much about it anymore, but you might want to > watch my video at > https://www.sqlalchemy.org/library.html#thesqlalchemysessionindepth > where I went through an amount of effort I would never have time for > these days in order to show what the general idea of it is. Also > note the Session concept is mostly from Hibernate, including the API > for merge() as well as cascades which I largely copied without deeply > considering it beyond the heavy thinking they did up front; the pycon > talk I did after several years of learning how the Session was > actually supposed to work which itself required that I redesigned half > the thing in version 0.5. > > > > > > -Lyla > > > > On Mon, Apr 29, 2019 at 1:04 PM Mike Bayer > wrote: > >> > >> On Mon, Apr 29, 2019 at 12:04 PM Lyla Fischer > wrote: > >> > > >> > Sorry about the language. I'll try to stick to "intuitive" and > "surprising" in the future. > >> > > >> > I guess that I am familiar enough with the concept that something > needs to persist in the database before it can be referenced in the > database that I was surprised when an object was already in the database > even when I didn't tell it to persist. > >> > > >> > This is especially true in the case of a one-to-many relationship, > where the head of a tassel can exist without any tassel threads. It just > happened to be the case that I added a tassel thread and... now it is in > the session?? Quoting from a previous email, for easy reference: > >> > """ > >> > my_head = Head(id="foobar") > >> > my_head = db_session.merge(my_head) > >> > db_session.commit() > >> > > >> > my_tassel_thread = TasselThread(head=my_head) > >> > > >> > "my_tassel_thread" is now in the Session due to the backref cascade, > >> > which I think I really might consider defaulting to False at some > >> > point. > >> > """ > >> > > >> > What would happen if I were editing an existing TasselThread, instead > of adding a new one? Would those changes be persisted as well, even though > I didn't tell them to explicitly? > >> > >> so the idea of "an existing TasselThread" in terms of the ORM means > >> you would have accessed it from the database, which means it would > >> have a primary key value and already be associated with that Session. > >> So you would expect that changes on this object are persisted. If > >> you expunged the object so that it became detached, then yes again the > >> cascade_backrefs thing gets in the way (or not, depending on how you > >> look at it) if you re-associate it with a persistent object. > >> > >> > > >> > I get that you are doing stuff in the backend that involves database > persistence in order to make the collection references work, but if it were > implemented using only foreign keys, then the head would have no way of > accessing it's tassel threads short of doing a table scan for it's own > primary key in the foreign key spot. I get that's massively inefficient, > and therefore not the way that you are doing things, > >> > >> if you don't use the backref feature at all, then it actually does > >> work that way.you can wait for flushes to happen, then expire > >> attributes on things once they are persistent. this way of working > >> will expose the persistence mechanics a lot more explicitly and > >> inconveniently though. > >> > >> > but I was thinking of SQL Alchemy relationship as being something > that very well could have been implemen
Re: [sqlalchemy] Possible regression?
Ah. I think that I have been thinking of a session as a cache instead of a transaction, which is incorrect. (It's similar because you have to worry about persistence in the face of failures, but not-similar, because you're not worried about communication with other processes. That is, a commit is not "expose this data to other cores/machines". A commit is a checkpoint to rollback to.) I was thinking that adding something to the session was taking something from a local and putting it into a cache, and then committing it was taking it from a cache and putting it into the database. Totally wrong abstraction, sorry. -Lyla On Mon, Apr 29, 2019 at 1:04 PM Mike Bayer wrote: > On Mon, Apr 29, 2019 at 12:04 PM Lyla Fischer wrote: > > > > Sorry about the language. I'll try to stick to "intuitive" and > "surprising" in the future. > > > > I guess that I am familiar enough with the concept that something needs > to persist in the database before it can be referenced in the database that > I was surprised when an object was already in the database even when I > didn't tell it to persist. > > > > This is especially true in the case of a one-to-many relationship, where > the head of a tassel can exist without any tassel threads. It just happened > to be the case that I added a tassel thread and... now it is in the > session?? Quoting from a previous email, for easy reference: > > """ > > my_head = Head(id="foobar") > > my_head = db_session.merge(my_head) > > db_session.commit() > > > > my_tassel_thread = TasselThread(head=my_head) > > > > "my_tassel_thread" is now in the Session due to the backref cascade, > > which I think I really might consider defaulting to False at some > > point. > > """ > > > > What would happen if I were editing an existing TasselThread, instead of > adding a new one? Would those changes be persisted as well, even though I > didn't tell them to explicitly? > > so the idea of "an existing TasselThread" in terms of the ORM means > you would have accessed it from the database, which means it would > have a primary key value and already be associated with that Session. > So you would expect that changes on this object are persisted. If > you expunged the object so that it became detached, then yes again the > cascade_backrefs thing gets in the way (or not, depending on how you > look at it) if you re-associate it with a persistent object. > > > > > I get that you are doing stuff in the backend that involves database > persistence in order to make the collection references work, but if it were > implemented using only foreign keys, then the head would have no way of > accessing it's tassel threads short of doing a table scan for it's own > primary key in the foreign key spot. I get that's massively inefficient, > and therefore not the way that you are doing things, > > if you don't use the backref feature at all, then it actually does > work that way.you can wait for flushes to happen, then expire > attributes on things once they are persistent. this way of working > will expose the persistence mechanics a lot more explicitly and > inconveniently though. > > > but I was thinking of SQL Alchemy relationship as being something that > very well could have been implemented without creating new things to > persist in the database beyond what is in the bare minimum of common > explanations of SQL-level one-to-many relationships, often used in intro > classes. > > what are the "new things to persist" you are referring towards ? > Are you referring to the fact that in order to set > TasselThread.some_foreign_key to a value, you need a Tassel object? > That is also optional. You can set foreign key values directly as > well, and there is no issue with that, except that it does not > automatically refresh the related relationships; you'd need to expire > them directly or wait until everything is expired on commit. The FAQ > entry > https://docs.sqlalchemy.org/en/13/faq/sessions.html#i-set-the-foo-id-attribute-on-my-instance-to-7-but-the-foo-attribute-is-still-none-shouldn-t-it-have-loaded-foo-with-id-7 > discusses this in detail. > > There is also a related recipe > https://github.com/sqlalchemy/sqlalchemy/wiki/ExpireRelationshipOnFKChange > (the FAQ has an apparently broken link to it) which expands upon the > "set foreign key attributes" way of working so that the related > objects *are* automatically refreshed immediately.There is at > least one major production-level application that uses this recipe, > which is why it's so complicated because
Re: [sqlalchemy] Possible regression?
Sorry about the language. I'll try to stick to "intuitive" and "surprising" in the future. I guess that I am familiar enough with the concept that something needs to persist in the database before it can be referenced in the database that I was surprised when an object was already in the database even when I didn't tell it to persist. This is especially true in the case of a one-to-many relationship, where the head of a tassel can exist without any tassel threads. It just happened to be the case that I added a tassel thread and... now it is in the session?? Quoting from a previous email, for easy reference: """ my_head = Head(id="foobar") my_head = db_session.merge(my_head) db_session.commit() my_tassel_thread = TasselThread(head=my_head) "my_tassel_thread" is now in the Session due to the backref cascade, which I think I really might consider defaulting to False at some point. """ What would happen if I were editing an existing TasselThread, instead of adding a new one? Would those changes be persisted as well, even though I didn't tell them to explicitly? I get that you are doing stuff in the backend that involves database persistence in order to make the collection references work, but if it were implemented using only foreign keys, then the head would have no way of accessing it's tassel threads short of doing a table scan for it's own primary key in the foreign key spot. I get that's massively inefficient, and therefore not the way that you are doing things, but I was thinking of SQL Alchemy relationship as being something that very well could have been implemented without creating new things to persist in the database beyond what is in the bare minimum of common explanations <https://stackoverflow.com/questions/1308339/create-a-one-to-many-relationship-using-sql-server> of SQL-level one-to-many relationships, often used in intro classes. Then the relationship just means that I don't have to do a table scan - the object just knows which TasselThreads are its own. That doesn't fit the common university model of the way foreign key only one-to-many relationships work, but I didn't think about it too hard. I was just thinking that "yes, of course you need to persist the head before persisting a tassel thread, because the tassel thread needs the id for its foreign key", while the reverse didn't occur to me. -Lyla On Mon, Apr 29, 2019 at 11:14 AM Mike Bayer wrote: > On Mon, Apr 29, 2019 at 7:22 AM Lyla Fischer wrote: > > > > If I can comment on what seems instinctive to someone who just recently > read the documentation, and made assumptions based off of it: My > understanding was that the relationship abstractions were supposed to be > about making foreign keys easier to keep track of, and being able to go > both ways on a relationship in a object-oriented way. The fact that there > were any implications at all for persistence was surprising to me, and it > took me a significant amount of time to understand that SQL Alchemy was > trying to do some magic behind the scenes when it came to persistence. I > support the idea of making cascade_backrefs=False by default, because it > keeps the separate objects separate, it makes me less nervous about > eventual performance (which is a constant concern when there is magic I > didn't intend happening in a library), and it's basically just working the > way that I expected things to work initially. I'm fine with dealing with > errors that might come about from the implications of updating of objects > independently. It seems like part of the responsibilities of dealing with > persistence, ever. > > I'm trying to gather the rationale that you are referring towards in > your comment, however I'm not able to work with the reason "magic I > didn't intend", because that reason doesn't actually say anything. > Of course every issue is about telling a library what your intent is, > and the library fulfullling that intent or not.But to refer to the > intents that you didn't expect as "magic" and the intents that you > *did* expect as "not magic", I guess the latter is implicit, only > illustrates how much the library is doing completely correctly such > that you aren't noticing it. > > The thing that relationship() does is coordinating Python objects that > are linked together in terms of a foreign key relationship in a > database.Then there is the backref concept, which expands on the > relationship concept, to more fully emulate the behavior of relational > database foreign keys with Python objects. When working completely > normally, SQLAlchemy produces Python object behavior as follows: > > object_a = ObjectA() > object_b = ObjectB() > object_a.some_collection.append(object_b) >
Re: [sqlalchemy] Possible regression?
If I can comment on what seems instinctive to someone who just recently read the documentation, and made assumptions based off of it: My understanding was that the relationship abstractions were supposed to be about making foreign keys easier to keep track of, and being able to go both ways on a relationship in a object-oriented way. The fact that there were any implications at all for persistence was surprising to me, and it took me a significant amount of time to understand that SQL Alchemy was trying to do some magic behind the scenes when it came to persistence. I support the idea of making cascade_backrefs=False by default, because it keeps the separate objects separate, it makes me less nervous about eventual performance (which is a constant concern when there is magic I didn't intend happening in a library), and it's basically just working the way that I expected things to work initially. I'm fine with dealing with errors that might come about from the implications of updating of objects independently. It seems like part of the responsibilities of dealing with persistence, ever. -Lyla On Sun, Apr 28, 2019 at 10:54 PM James Fennell wrote: > Thanks for the explanation Mike! Seeing it now, I actually think there’s a > decent reason to want the current backerefs: > > My understanding is that with session.merge in SQL Alchemy it’s possible > to draw a very clean line between entities that are persisted (or about to > be persisted on the next flush) and entities which will never be persisted. > This is owing to the design choice whereby SQL Alchemy doesn’t persist the > entity you pass into the merge; instead, that is kept alone and a new > entity is created. > > With this in mind, there are two ways to see Lyla’s example. > > One way: as soon as the tassel_thread was related to the persisted my_head > (persisted because of the line my_head=session.merge(my_head)) then > tassel_thread should be seen as in the session already. In this view, the > merge is necessary and possibly error-prone, as here. > > Another way: instead of assigning my_head=session.merge(my_head), keep the > unpersisted head around with say persisted_head = session.merge(my_head). > Then relating the new tassel_thread to my_head won’t add it to the session. > To get a record into the DB, then do a session.merge on it - everything > works correctly this way. > > > In both cases, there is the idea of a persisted object graph and a > distinct unpersisted object graph. Once you relate a new entity to > something in the persisted object graph, it becomes persistent. > > -- > 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. > --- > You received this message because you are subscribed to a topic in the > Google Groups "sqlalchemy" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/sqlalchemy/oVVdbCzsNQg/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > sqlalchemy+unsubscr...@googlegroups.com. > To post to this group, send email to sqlalchemy@googlegroups.com. > Visit this group at https://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. > -- 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. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at https://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
Re: [sqlalchemy] Possible regression?
gt; > I recently came across some confusing behavior in relations and
> cascading using sqllite, and I was hoping that I might get some help
> explaining what the behavior is here. I put together a minimum failing
> script here. I'm trying to commit one instance of each of two classes, but
> what ends up happening is that I commit two copies of the many part of a
> one-to-many relation. I suspect that this has something to do with
> cascading, but I found a bug report for similar behavior that claims to
> have been fixed several years ago, and I'm wondering if there was some kind
> of regression? I'm running SQLAlchemy 1.3.1 on Ubuntu and I'm still using
> sqllite at this stage of development.
> >> > >
> >> > > from sqlalchemy import create_engine
> >> > > from sqlalchemy.orm import scoped_session, sessionmaker
> >> > > from sqlalchemy.ext.declarative import declarative_base
> >> > > from sqlalchemy import Column, Integer, String, Text, ForeignKey
> >> > > from sqlalchemy.orm import relationship
> >> > >
> >> > > import os
> >> > >
> >> > > db_engine = create_engine('sqlite:///sample.db',
> convert_unicode=True)
> >> > > db_session = scoped_session(sessionmaker(autocommit=False,
> >> > > autoflush=False,
> >> > > bind=db_engine))
> >> > >
> >> > > Base = declarative_base()
> >> > > Base.query = db_session.query_property()
> >> > >
> >> > > class Head(Base):
> >> > > __tablename__ = 'head'
> >> > > id = Column(String, primary_key=True)
> >> > > tassel_threads = relationship("TasselThread",
> back_populates="head")
> >> > > def __init__(self, id):
> >> > > self.id=id
> >> > >
> >> > > class TasselThread(Base):
> >> > > __tablename__ = 'tassel_thread'
> >> > > id = Column(Integer, primary_key=True)
> >> > > head_id = Column(Integer, ForeignKey('head.id'),
> nullable=False)
> >> > > head = relationship("Head", back_populates="tassel_threads")
> >> > > def __init__(self, head):
> >> > > self.head = head
> >> > >
> >> > > def init_db():
> >> > > Base.metadata.create_all(bind=db_engine)
> >> > >
> >> > >
> >> > > def do_db_work():
> >> > >
> >> > > my_head = Head(id="foobar")
> >> > > my_head = db_session.merge(my_head)
> >> > > db_session.commit()
> >> > >
> >> > > my_tassel_thread = TasselThread(head=my_head)
> >> > > db_session.merge(my_tassel_thread)
> >> > > db_session.commit()
> >> > >
> >> > >
> >> > > if os.path.exists("sample_data.db"):
> >> > > os.remove("sample_data.db")
> >> > > init_db()
> >> > > do_db_work()
> >> > > a = db_session.query(TasselThread).all()
> >> > > print(len(a))
> >> > > # output: 2, should be 1
> >> > >
> >> > > Thanks for any help you might be able to provide!
> >> > >
> >> > > -Lyla Fischer
> >> > >
> >> > > --
> >> > > 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.
> >> > > ---
> >> > > You received this message because you are subscribed to the Google
> Groups "sqlalchemy" group.
> >> > > To unsubscribe from this group and stop receiving emails from it,
> send an email to sqlal...@googlegroups.com.
> >> > > To post to this group, send email to sqlal...@googlegroups.com.
> >> > > Visit this group at https://groups.google.com/group/sqlalchemy.
> >> > > For more options, visit https://groups.google.com/d/optout.
> >
> > --
> > 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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>
> --
> 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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--
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
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