Hi Michael,
It really works like magic! I mean the refreshing part. What exactly
happens when refreshing a1.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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