On Aug 8, 2013, at 7:47 PM, askel <dummy...@mail.ru> wrote:
> Hello everyone,
>
> I've been using SQLAlchemy for a while and it's been a pleasure. But
> sometimes I feel I can get a little more from it or do something more
> properly. For instance, imagine two tables with one-to-one relationship bosth
> sharing the same primary key and second table containing primary key only
> (please don't ask me why it wasn't made just a nullable boolean column to the
> first table).
>
> Base = declarative_base()
>
> class Item(Base):
> __table_name__ = 'items'
>
> i_id = Column(Integer, primary_key=True)
> # other columns omitted
>
> special_items = Table('special_items', Base.metadata,
> Column('i_id', Integer, ForeignKey('items.i_id'), primary_key=True),
> # no any other columns
> )
>
> Item.is_special = exists([1]).where(Item.i_id == special_items.c.i_id)
>
> And sometimes I need to use that is_special property in query filter as the
> following:
>
> query(Item).filter(and_(Item.is_special, ...))
>
> which works well in queries -- exists sub-query is added if/whenever
> is_special is mentioned in filter.
>
> What I would also like to see is is_special becoming a boolean property of
> Items instance. And it would be awesome to be able to assign to it so that it
> fires correct insert statement when session is flushed.
>
> To make first thing happen it sounds like I need to use hybrid_property but I
> couldn't figure out what is_special method should return when it's accessed
> as instance property. More over that property should be lazy, i. e. I don't
> want every query have that exists sub-query UNLESS it's explicitly used in
> filter or any other way.
>
> class Item(Base):
> ...
> @hybrid_property
> def is_special(self):
> return ???
>
> @is_special.statement
> def is_special(cls):
> return exists([1]).where(cls.i_id == special_items.c.i_id)
>
>
> I'd greatly appreciate if anybody gives me tip on how to make above happen
> (may be partially). It's not like I cannot live without it though.seems like you'd create Item.special_items as a relationship() to the special_items table (using some class), then the python is_special() just says return bool(self.special_items). If you create the relationship() with lazy=dynamic you could just say bool(self.special_items.count()).
signature.asc
Description: Message signed with OpenPGP using GPGMail
