I am finding myself doing a fair amount of copy-and-paste in the data
model I'm currently working on, and I'd like to reduce that if
possible. For example, I have several different types of objects that
have names belonging to namespaces. So every such table gets
boilerplate looking like this (using declarative):
namespace_id = Column(Integer, ForeignKey(Namespace.id),
nullable=False)
namespace = relationship(Namespace, backref='widgets')
name = Column(Unicode(256), nullable=False)
__table_args__ = (
UniqueConstraint(namespace_id, name),
{})
In the above example, the only thing which changes from class to class
is the backref='widgets' part. As another example, I would like to be
able to attach notes to various kinds of objects, and since the
objects are not related by inheritance, I think the best way is to
create a separate notes table for each class of object. So for
example a Widget class (mapped to widgets table) would lead to a
WidgetNote class (mapped to widget_notes). (The alternative, having a
single notes table, requires all notable objects to be related by
polymorphic inheritance, which as I said isn't really a great idea in
my case.) Ideally, I could just stick a decorator on the Widget class
and have the notes handled automagically.
I am thinking that metaclasses are the appropriate thing here. I have
never delved deeply into metaclasses, though, and I know that
SqlAlchemy is already doing a lot of metaclass magic, and I worry that
I may screw something up. And I really need several different types
of metaclasses that can be combined, so I need some kind of metaclass
mix-in, and that all sounds very disaster-prone. Am I barking up the
wrong tree here? Can anybody point me to some examples of metaclasses
being used for something like this?
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