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).
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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