Got it! It works, that's the main thing!
On Sun, Mar 16, 2014 at 8:24 PM, Michael Bayer <mike...@zzzcomputing.com>wrote:
>
> a1.bs only loads something if there's nothing loaded already, or if the
> value was "expired".
>
> usually, as long as you keep objects associated with a Session, there's no
> need to worry about it as things load as they are needed. but in that
> specific example, I'm moving an object from one session to another and
> trying to trip a condition that only shows up if the object were modified
> while it's "detached". it isn't a usual thing.
>
>
>
> On Mar 16, 2014, at 10:45 PM, Bao Niu <niuba...@gmail.com> wrote:
>
> Hi Michael,
>
> It really works like magic! I mean the refreshing part. What exactly
> happens when refreshinga1.bs? Does it actually replace the old b1 inside
> the collection with a newer one? I can't wrap my mind around it. And more
> importantly, does it mean I should *refresh* every attributes before close
> a session, in order to carry their attribute over to the next session?
> What is the reason not making such *refreshing* automatic by default???
>
> Sorry for bringing up another question here.
>
>
> On Sun, Mar 16, 2014 at 10:58 AM, Michael Bayer <mike...@zzzcomputing.com>
> wrote:
>
>>
>> On Mar 15, 2014, at 7:41 PM, Bao Niu <niuba...@gmail.com> wrote:
>>
>> Thanks a lot Michael! Just a trivial question here, I noticed in your
>> first reply you used:
>> # refresh a1.bs
>>
>> Why do we need to refresh it? I tried it in my terminal and it doesn't
>> emit any sql. Is this one if those secret techniques that differentiate a
>> sqlalchemy ninja and a newbie?;)
>>
>>
>> it should emit SQL, because the sess.commit() call above has expired the
>> collection. in my script it fails down on "a1.bs.remove()" if i don't call
>> that first.
>>
>>
>>
>>
>> On Mar 15, 2014 8:31 AM, "Michael Bayer" <mike...@zzzcomputing.com>
>> wrote:
>>
>>> 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 <niuba...@gmail.com> 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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