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.
- Alex
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