[sqlalchemy] Re: Possible regression?

[James Fennell]

It seems to be related to the cascades happening recursively. The merge 
cascade goes from the tassel thread to the head, and then again down from 
the head to the tassel thread - which is kind of strange, I would expect 
the cascades to only visit each node in the object graph at most once. The 
second cascade is when the second tassel thread is created.

There are a couple of ways that I can get this to work:

   - Instead of setting head when creating the tassel thread, set head_id. 
   This avoids the first cascade being triggered.
   - Add cascade=None on the relationship in the Head to avoid the second 
   cascade being triggered. However the merge operation on the tassel thread 
   now returns a warning that the "add operation along 'Head.tassel_threads' 
   will not proceed"
   - The your second merge, instead of merging in the tassel_thread, merge 
   in the head again.

Btw, I'm using SQL Alchemy 1.2.16 and the thing you're seeing is still 
repro-able.

On Sunday, 28 April 2019 08:56:40 UTC-4, lyla...@gmail.com 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 = 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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http://www.sqlalchemy.org/

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[sqlalchemy] Re: Possible regression?

[peter . m . schutt]

Hi Lyla,

I notice Head.id is defined as String:

>     id = Column(String, primary_key=True)

While TasselThread.head_id is defined as Integer:

>     head_id = Column(Integer, ForeignKey('head.id'), nullable=False)

Could it be that causes the merge to not recognize the existing instance and a 
new one is created?

-- 
SQLAlchemy - 
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

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