On Fri, Nov 13, 2015 at 8:16 AM, Yegor Roganov <yegor....@gmail.com> wrote:
> Suppose we have a 1 <-> N relation between users and cities (meaning that
> every user is related with one city). For our domain model "User" we want
> to define a method "lives_in_london". Code looks like this:
>
> class User(Base):
> id = ...
> city_id = ...
> city = relationship("City")
> def lives_in_london(self):
> return self.city.name == 'London'
>
> class City(Base):
> id = ...
> name = Column(String)
>
>
> The problem with this code is that "lives_in_london" method is a leaky
> abstraction, its client must keep in mind that it may issue a DB query.
> It's okay as a one-off thing, but will case problems if used in a loop.
> So to be used efficiently, clients must know to preload the "city"
> relationship.
>
> I know it's a contrived example, but the general question is how to define
> domain methods that need to access relations. I also know that the question
> is intrinsic to all ORM, but maybe SQLAlchemy could offer some support.
>
> I would appreciate any ideas.
>
>
I'm probably suffering from a lack of imagination here, but this seems like
an impossible task. How could SQLAlchemy know ahead of time whether you are
going to access the "city" property on any given user? And whether you're
going to access it on just one user, or on lots of users in a loop?
If that's not what you meant, perhaps you could suggest an API that would
do what you wanted?
Simon
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