Hello everyone.
I was wondering if it's possible to inherit a custom collection to
create another custom collection.
A few days ago I was trying to use my own class as a custom_collection
(http://groups.google.com/group/sqlalchemy/msg/ba1c64c3d227f586).
Thanks to Michael Bayer I was able to do it, but now I would like to
go one step further, and inherit my custom collection to create
another custom collection.
To simplify a little what I asked in the other message, let's say I have a:
def ClassA(declarativeBase):
__tablename__ = "aes"
id = Column("id", Integer, primary_key=True)
_whatever = Column("type", String(64))
def __init__(self):
self._whatever = "whatever"
Then I have my custom collection for instances of "ClassA":
def ContainerOfA(dict):
__emulates__ = set
def __init__(self):
self._field = "I'm a great... awesom! container"
#I also defined the appender, remover and iterator
@collection.iterator
def __iter__(self):
return self.itervalues()
@collection.appender
def append(self, item):
self[item.getUniqueHash()] = item
@collection.remover
def remove(self, item):
if item.getUniqueHash() in self.keys():
del self[item.getUniqueHash()]
And then I was happily able to use it in any relationships:
def YetAnotherClass(declarativeBase):
id = Column("id", Integer, primary_key=True)
classesA = relationship("ClassA",
uselist=True,
secondary="intermediate_table",
collection_class=lambda: ContainerOfA(),
cascade="all, delete, delete-orphan",
single_parent=True
)
Now I needed to extend "ClassA" in a "Class B" and "ContainerOfA" in
"ContainerOfB". I added the polymorphic stuff to "ClassA" and "ClassB"
to create a joined table inheritance, as detailed in
http://www.sqlalchemy.org/docs/orm/inheritance.html#joined-table-inheritance
. (it seems to be working fine, that's why I am not completely
detailing it here)
def ClassB(ClassA):
__tablename__ = "bs" #Sorry for that
__mapper_args__ = {'polymorphic_identity': 'ClassB'}
id = Column("id", Integer, ForeignKey('aes.id'), primary_key=True)
def __init__(self):
self._anotherWhatever = "another whatever"
def ContainerOfB(ContainerOfA):
def __init__(self):
super(ContainerOfB, self).__init__()
def anotherMethodOnlyForBInstances(self):
# do interesting stuff for B classes
Then I tried to use it in a relationship:
def YetYetAnotherClass(declarativeBase):
id = Column("id", Integer, primary_key=True)
classesB = relationship("ClassB",
uselist=True,
secondary="another_intermediate_table",
collection_class=lambda: ContainerOfB(),
cascade="all, delete, delete-orphan",
single_parent=True
)
But when I tried to append a "ClassB" instance through the
relationship detailed above, I got this exception:
>> Type ContainerOfB must elect an appender method to be a collection class
I thought... "ok, ok... let's just explicitly add the 'appender' to
the ContainerOfB class... The only thing I need to do is calling the
appender of the super class, anyway... no biggie" and so I did:
def ContainerOfB(ContainerOfA):
# [ . . . ] #
@collection.appender
def append(self, classBInstance):
return super(ContainerOfB, self).append(classBInstance)
But then... another exception when I tried to add an instance of ClassB():
>> InvalidRequestError: Instance <ClassB at 0xba9726c> is already associated
>> with an instance of <class 'mylibraries.classesTest.YetYetAnotherClass'> via
>> its YetYetAnotherClass.classesB attribute, and is only allowed a single
>> parent.
Well... I need the cascade to properly delete the items
(http://www.sqlalchemy.org/docs/orm/session.html#deleting) and in
order to use that, I need the single_parent = True.
Then funny thing is that if I totally rewrite the appender method in
ContainerOfB:
def ContainerOfB(ContainerOfA):
# [ . . . ] #
@collection.appender
def append(self, classBInstance):
# write here the exact same code than ContainerOfA changing
# the reference to the "item" parameter by "classBInstance"
# (that's the only difference)
then everything is working fine. I have made some more tests, and the
inheritance ClassA <- ClassB seems to be working fine. In said tests I
removed the "cascade" and the "single_parent" parameters of the
"classesB" relationship. By doing that, I was able to insert instances
of ClassB in the classesB container and all the information was
properly stored in the database (the polymorphic identity was added
properly, the foreign key of the ClassB() instance was properly
pointing to its "parent" class "ClassA"'s id, all the fields were
there... perfect!). The problem seems to come when I try to use the
"single_parent = True" with my inherited ContainerOfB class.
As far as I can tell (and I may be totally wrong) it seems to me that
somehow, calling the "super - appender" appends the instance twice. I
thought that because rewriting the whole "append" method in my
ContainerOfB fixed the problem. But that is not "perfect". In regular
OOP I don't need to do that. Is there any way to fix that? Am I
missing something? (sorry... let me rephrase it: "What am I missing?",
because, newbie as I am, I'm sure I'm doing something wrong)
Reading the custom collections chapter
(http://www.sqlalchemy.org/docs/orm/collections.html#instrumentation-and-custom-types),
I found:
"""
However, it is important to note that the instrumentation process
_will_ modify the type, adding decorators around methods
automatically.
"""
I don't know if this is what is affecting my code, but if so, I
haven't been able to find a better workaround than rewriting the
"append" method.
Any hint will be appreciated! Thank you in advance!
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