Simon, thanks for your reply.
As I said, I'm not sure that this can be solved on the ORM-level, rather
I'm interested in how others solve similar issues. In a small project it's
possible to remember what each method is doing internally, but in large
ones with dozens of programmers it can be a real problem, hence my question.
On Friday, November 13, 2015 at 12:21:44 PM UTC+2, Simon King wrote:
>
> On Fri, Nov 13, 2015 at 8:16 AM, Yegor Roganov <yego...@gmail.com
> <javascript:>> 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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