you have every reason to be confused by that paragraph, which is using way too
much terminology to express what's important there. at some point, we had to
add a behavior which I thought would be confusing to people, so that paragraph
tries badly to explain what it is. I should replace it with just a simple
sentence and an example. Here's the example:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class A(Base):
__tablename__ = 'a'
id = Column(Integer, primary_key=True)
bs = relationship("B")
class B(Base):
__tablename__ = 'b'
id = Column(Integer, primary_key=True)
a_id = Column(Integer, ForeignKey('a.id'))
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
sess = Session(e)
a1 = A()
b1 = B()
a1.bs = [b1]
sess.add(a1)
sess.commit()
a1.bs # refresh a1.bs
sess.close() # close out - sess is no longer associated with a1, b1
# all new session
sess2 = Session(e)
a1.bs.remove(b1)
sess2.add(a1)
# b1 was removed from a1.bs, but
# is in sess2 anyway! surprising!
assert b1 in sess2
# because we need it for the flush, it's still here:
from sqlalchemy import inspect
print inspect(a1).attrs.bs.history.deleted
On Mar 15, 2014, at 5:26 AM, Bao Niu <[email protected]> wrote:
> I've read this paragraph
> (http://docs.sqlalchemy.org/en/latest/orm/session.html#unitofwork-cascades)
> many many times and still can't think of a practical example of what is being
> discussed.
>
> save-update cascade also cascades the pending history of the target
> attribute, meaning that objects which were removed from a scalar or
> collection attribute whose changes have not yet been flushed are also placed
> into the target session. This is because they may have foreign key attributes
> present which will need to be updated to no longer refer to the parent.
>
> I don't think my English is the main stumbling block here because I
> understand the meaning of each word, but as soon as I'm putting them together
> I'm completely lost. Could someone give a simple example here to illustrate
> the main point in this paragraph please? Highly appreciated. Thanks.
>
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