Hi Michael,
I am trying to run the alternative you described in the article, but
the following code is most likely from an old version of SA a don't
know how to update (I am working with 6.6):
mapper = class_mapper(cls)
table = mapper.local_table
mapper.add_property(attr_name, relationship(GenericAssoc,
backref=backref('_backref_%s' % table.name, uselist=False)))
class_mapper is unknown or moved.
What does it do/how can I fix this?
Cheers, Lars
On Apr 6, 10:18 pm, Michael Bayer <mike...@zzzcomputing.com> wrote:
> On Apr 6, 2011, at 5:43 AM, farcat wrote:
>
> > Hello,
>
> > I am experimenting with a pattern where records hold the table name
> > and record id of the next record in any other table, chaining records
> > in different tables. This works, but I can't figure out how to clean
> > op references to the next record in another table when I delete a
> > record (the pattern does not use foreign keys in the normal sense).
>
> .. in that it doesn't use foreign keys. Since you're working against the
> relational database's supported patterns, you'd need to roll the deletion of
> related rows yourself. The pattern is also called a "polymorphic
> association" and I blogged about it years ago here:
> http://techspot.zzzeek.org/2007/05/29/polymorphic-associations-with-s....
>
>
>
>
>
>
>
> > The code is:
>
> > =====================================
>
> > from sqlalchemy import *
> > from sqlalchemy.orm.session import sessionmaker
> > from sqlalchemy.ext.declarative import declarative_base,
> > declared_attr, DeclarativeMeta
> > #--------------------------------------------------------------------------
> > -
> > Base = declarative_base()
> > reg = dict()
> > engine = create_engine('sqlite:///:memory:', echo=False)
> > Session = sessionmaker(bind = engine)
> > #--------------------------------------------------------------------------
> > -
>
> > class chainmeta(DeclarativeMeta):
> > #--------------------------------------------------------------------------
> > -
> > class Base(object):
> > session = Session()
> > @declared_attr
> > def __tablename__(cls):
> > return cls.__name__
>
> > id = Column(Integer, primary_key = True)
> > next_table = Column(String(64))
> > next_id = Column(Integer) #in table with name stored in
> > next_table!
>
> > def __init__(self, data, next = None):
> > self.data = data
> > self.prev = None
> > self.next = next
> > self.session.add(self)
> > self.session.flush()
>
> > def _getnext(self):
> > if self.next_table and self.next_id:
>
> > return
> > self.session.query(reg[self.next_table]).filter(self.next_id ==
> > reg[self.next_table].id).one()
> > else: return None
>
> > def _setnext(self, next):
> > if next:
> > if self.next:
> > self.next.prev = None
> > self.next_table = next.__tablename__
> > self.next_id = next.id
> > next.prev = self
> > elif self.next:
> > self.next.prev = None
> > self.next_table = None
> > self.next_id = None
>
> > def _delnext(self):
> > self.next.prev = None
> > self.next_table = None
> > self.next_id = None
>
> > next = property(_getnext, _setnext, _delnext)
>
> > def __repr__(self):
> > out = "type: " + type(self).__name__ + "["
> > for name in self.__dict__:
> > out += name + ", "
> > out += "]"
> > return out
> > #--------------------------------------------------------------------------
> > -
> > def __new__(mcls, name, coltype):
> > return DeclarativeMeta.__new__(mcls, name, (chainmeta.Base,
> > Base),{"data": Column(coltype, nullable = False)})
> > def __init__(cls, name, coltype):
> > reg[name] = cls
> > return DeclarativeMeta.__init__(cls, name, (chainmeta.Base,
> > Base),{})
> > #--------------------------------------------------------------------------
> > -
> > if __name__ == '__main__':
> > Base.metadata.drop_all(engine)
> > session = chainmeta.Base.session = Session()
>
> > Ni = chainmeta("Ni", Integer)
> > Nb = chainmeta("Nb", Boolean)
> > Nt = chainmeta("Nt", String(200))
> > Base.metadata.create_all(engine)
>
> > ni1 = Ni(5)
> > ni2 = Ni(12)
> > nb1 = Nb(True)
> > nb2 = Nb(False)
> > nt1 = Nt("text in nt1")
> > nt2 = Nt("text in nt2")
> > ni1.next = ni2
> > ni2.next = nb1
> > nb1.next = nb2
> > nb2.next = nt1
> > nt1.next = nt2
> > nt2.next = ni1 #circular
> > print "OBJECTS"
> > n = ni1
> > count = 0
> > print "nexts: ................."
> > while n and count < 10:
> > print n.data
> > count += 1
> > n = n.next
> > n = ni1
> > count = 0
> > print "prevs: ................."
> > while n and count < 10:
> > print n.data
> > count += 1
> > n = n.prev
> > print
> > "--------------------------------------------------------------------------
> > ---------"
> > nts = session.query(Nt).all()
> > print "QUERIES"
> > for nt in nts:
> > print nt.data
> > print "+++++++++++++++++++++"
> > print session.query(Ni).filter(Ni.next_id ==
> > nb1.id).first().data
>
> > =====================================
>
> > This might seem to have no reasonable us case, but it is something I
> > want to use in a more complicated pattern later on. Basically the
> > question is, how can I remove a record and have no next or prev
> > pointing to it in other objects or records (without adding some sort
> > of external controller)?
>
> > Cheers, Lars
>
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