Re: [sqlalchemy] Possible regression?

[lylafisch]

Thanks for the quick responses, and for adding the warning for others! 

I tried adding the lines as suggested (

head = relationship("Head", backref="tassel_threads", cascade_backrefs=False
)

, but I keep getting the same error. I also tried cascade=None, as James 
suggested, but that seemed to mess up the ability of the relationships to 
get at each other's primary keys or establish relationships with each 
other. I ended up solving the issue by removing some of the merges. That 
means that I'm depending on the cascading to get everything into the 
database and linked up appropriately, andt I might need to spend some time 
studying exactly how cascading works in order to be able to reliably use 
the relationship feature in the future. 

-Lyla

On Sunday, April 28, 2019 at 12:46:06 PM UTC-4, Mike Bayer wrote:
>
> the next 1.3 release will include a warning for this case, whether or 
> not the primary key on the transient object is set up or not; it does 
> a double insert in any case otherwise.   It's just when the primary 
> key is already present, the double insert fails on the primary key 
> constraint. 
>
> https://github.com/sqlalchemy/sqlalchemy/issues/4647 
>
>
> On Sun, Apr 28, 2019 at 12:26 PM Mike Bayer  > wrote: 
> > 
> > Hi there, 
> > 
> > I appreciate everyone chiming in to look at this! 
> > 
> > However, it's something simple and has to do with one of the API 
> > quirks that we have to decide if we want to keep long term. 
> > Background is at 
> > https://docs.sqlalchemy.org/en/13/orm/cascades.html#backref-cascade. 
> > 
> > Basically, when you do this: 
> > 
> > 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.   it also has no primary key value yet.  So when you merge() 
> > it, it gets put in a second time, again with no primary key. 
> > 
> > Another thing that makes this really bad on my part, is that if you 
> > flush the session before the merge, then my_tassel_thread gets a new 
> > primary key, then the merge is of itself and it works.   This is bad 
> > because it suggests merge() should be calling flush() automatically, 
> > but im not sure that's a good idea in the bigger scheme of things. 
> > 
> > Short answer, set up the relationships like: 
> > 
> > tassel_threads = relationship("TasselThread", 
> > back_populates="head", cascade_backrefs=False) 
> > 
> > # ... 
> > 
> > head = relationship("Head", back_populates="tassel_threads", 
> > cascade_backrefs=False) 
> > 
> > 
> > and then my_tassel_thread stays out of the Session. 
> > 
> > Also: 
> > 
> > > db_engine = create_engine('sqlite:///sample.db', convert_unicode=True) 
> > 
> > don't use convert_unicode, it's deprecated, has no purpose in the 
> > modern Python ecosystem, and is going away. SQLite in particular 
> > is a fully Python unicode backend that's impossible to get a plain 
> > string out of. 
> > 
> > 
> > 
> > 
> > On Sun, Apr 28, 2019 at 8:56 AM > 
> wrote: 
> > > 
> > > Hi! 
> > > 
> > > 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 = 

[sqlalchemy] Possible regression?

[lylafisch]

Hi!

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

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