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On Dec 5, 2011, at 11:58 AM, Daniel Nouri wrote:
> Say I have a class that derives from a Node class that has a 'parent'
> relationship (using 'parent_id' and 'id' fields):
>
> from sqlalchemy.ext.hybrid import hybrid_property
>
> class MyClass(Node):
> @hybrid_property
> def grandparent(self):
> return self.parent.parent
>
> @grandparent.expression
> def grandparent(cls):
> pass # hmm?
>
> How would I go about the implementing the grandparent hybrid
> expression? What I have is a boolean clause list with two comparisons
> of 'parent_id' and 'id' using aliases (not included in the example).
> I was hoping I could return that but it's not what it expects.
>
> Thanks for your help.
yeah so this is where @hybrid can....not really do the whole thing.
Basically when you use a hybrid on a relationship, the @expression version
returns whatever it is that's most useful, given the context that you'd be
using it within. It of course cannot imply the automatic generation of join()
or anything like that, which is a manual thing...unless you had @expression
return a function that would process a given Query, which is pretty nasty and
not really helpful.
So in this case, I'm not even sure @expression can return a concept that is
meaningfully the "grandparent" - usually the best it can do is return
"cls.parent", but in this case that directly contradicts that it's called
"grandparent".
that would look like:
q = session.query(MyClass).\
join(parent_alias, MyClass.grandparent).\
filter(parent_alias.grandparent == some_object)
which is nasty because .grandparent is the same as .parent, so don't do that.
If it returned an aliased(cls).parent, then that alias object would need to be
memoized so that you can refer to it multiple times in the query (which is
possible). That would make the usage something like:
parent _alias = aliased(MyClass)
q = session.query(MyClass).\
join(parent_alias, MyClass.parent).\
join(MyClass.grandparent, parent_alias.parent).\
filter(MyClass.grandparent == some_node)
which also makes little sense, so....probably don't do that either :).
another awful thought I had, return a query option that runs it through
join(..., aliased=True):
q = session.query(MyClass).\
options(MyClass.grandparent).\
filter_by(id=id_of_my_grandparent)
I'd need to enhance options() a bit, or provide some other method to Query like
query.with_transformation(fn) that runs the query through a given callable.
the query above is completely opaque and pretty much can do just the one thing
it's been set up to do, not very useful :)
or if one were to really go nuts with that approach and produce a comparator
with an __eq__() method, maybe even:
session.query(MyClass).with_transformation(MyClass.grandparent==some_parent)
but I consider that to be kind of a novelty. Or maybe not,
with_transformation() might be a handy method to add in any case.
so yeah @expression.grandparent is not really very useful given the explicit
nature of the Query object, unless you seek to build tricks that build up the
Query behind the scenes.
> --
> http://danielnouri.org
>
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