On Sep 30, 2014, at 4:48 PM, katie louise <[email protected]> wrote:
> SQLAlchemy newbie here.
>
> I'm trying to define a model (subclassed by another model) that represents a
> subset of table data of the parent model. Specifically, I want the subclass
> to map the most recent row for a given ID.
>
> For example, suppose I have the following model:
>
> class AddressHistory(Base):
> __table__ = 'address_table'
>
> date = Column(Date, index=True, nullable=False)
> id = Column(BigInteger, primary_key=True)
> street = Column(String(2000))
> city = Column(String(2000))
> state = Column(String(2000))
> zip = Column(Integer)
> What I want to do is define a subclass of this model which represents the
> most recent address record:
>
> class MostRecentAddress(Address):
> """
> Represents a row in AddressHistory with the most recent date for a given
> id.
> """
> Is there some sort of subquery I can pass to the mapper_args ? Maybe a
> polymorphic_identity? Or could I create a separate view and have the subclass
> read from that?
>
>
Usually a query for "most recent X" involves grouping on a column that's not
the primary key. But if you want the most recent address across all primary
keys, that's exactly one row. I'm not really sure what it means to "map" to
exactly one row.
If you're looking for some pattern that gives you some fixed criteria, just use
a hybrid:
class AddressHistory(Base):
# ...
@hybrid_property
def max_address(cls):
return cls.id ==
select([AddressHistory.id]).group_by(AddressHistory.id).having(func.max(AddressHistory.date)
== AddressHistory.date)
max_address =
session.query(AddressHistory).filter(AddressHistory.max_address).first()
>
> Thanks!
>
>
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