On Tue, Jan 15, 2019 at 6:33 PM Daniel Leon <daniel19001...@gmail.com> wrote:
>
> from sqlalchemy import Column, ForeignKey, Integer, String, create_engine
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.ext.declarative.base import _declarative_constructor
> from sqlalchemy.orm import Session, relationship, sessionmaker
> from sqlalchemy.orm.dynamic import AppenderQuery
>
>
> class BaseBase(object):
>
> def __init__(self, **kwargs):
> _declarative_constructor(self, **kwargs)
> session.add(self)
> session.commit()
>
>
> Base = declarative_base(cls=BaseBase, constructor=BaseBase.__init__)
>
>
> class Node(Base):
>
> __tablename__ = 'node'
>
> id = Column(Integer, primary_key=True)
> label = Column(String)
> parent_id = Column(Integer, ForeignKey('node.id'))
> parent = relationship('Node', remote_side=id, back_populates='children')
> children = relationship('Node', back_populates='parent')
>
> def __repr__(self):
> return self.label
>
>
> if __name__ == '__main__':
> engine = create_engine('sqlite://', echo=True)
> Base.metadata.create_all(engine)
>
> Session = sessionmaker(bind=engine, expire_on_commit=False)
> session = Session()
>
> node = Node(label='root')
> # Accessing children for first time so query is made
> print node.children
> child1 = Node(label='child1', parent_id=node.id)
> # Accessing children again so no query is made and child1 is not there
> print node.children
> child2 = Node(label='child2', parent=node)
> # Accessing children again so no query is made and child1 is not there,
> but child2 is there
> print node.children
>
> In my MCVE I have expire_on_commit=False. When I access node.children for the
> first time, it makes that query. When I create child1 specifying parent_id,
> it doesn't update node.children.
this is expected behavior
>
> I am converting a codebase from SQLObject, which queries every time you
> access many-to-one field. With SQLAlchemy I know I can specify lazy='dynamic'
> in children relationship, but that returns a query object so I'd have to do
> node.children.all().
if you'd like foreign key updates to result in a flush, try out the
recipe at
https://github.com/sqlalchemy/sqlalchemy/wiki/ExpireRelationshipOnFKChange
which should at this point cover every contingency. but of course
long term you want to migrate off of that pattern as I don't know how
far the above recipe can be pushed.
>
> --
> 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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