On 03/29/2016 06:32 PM, Jonathan Vanasco wrote:
I'm in the processes of open-sourcing a mini-project and need to support
sqlite -- so I'd like to be a bit easier on DB operations.
The following use-case is puzzling me:
I want to view a record on TableA, which will show the last 5
related objects in TableB and a 'see all' link.
why can't you build a relationship() which has its primaryjoin set up to
do this? You can do it w/ a correlated subquery.
That is, unless you want this relationship() to load differently up
front with options. if you just want the limit() to change, put a
bindparam() inside of limit() and then use this approach:
https://bitbucket.org/zzzeek/sqlalchemy/wiki/UsageRecipes/GlobalFilter
It's represented in a one-to-many relationship:
import sqlalchemy as sa
class TableA(Base):
id = sa.Column(sa.Integer, primary_key=True)
rel_b = sa.orm.relationship('TableB', primaryjoin='TableA.id==
TableB.id_a', order_by='TableB.id.desc()')
class TableB(Base):
id = sa.Column(sa.Integer, primary_key=True)
id_a = sa.Column(sa.Integer, sa.ForeignKey("table_a.id"),
nullable=True)
I looked into `lazy='dynamic'` but ran into a few caveats:
* it is only an 'appendquery' until you iterate the relationship.
otherwise you need to call `.limit(n).all()`
* it doesn't cache the value. so if you iterate `.limit(5)` twice in a
row, it hits the db twice in a row.
* it doesn't support eagerloading
I was thinking about extending TableA with a dynamic version of the
property that I can just 'touch' during my load phase, and then
eagerload all the `rel_b` associations.
@property
def rel_b__latest(self):
if self._rel_b__latest is None:
self._rel_b__latest =
self.rel_b_dynamic.options(joinedload()..).limit(5).all()
return self._rel_b__latest
Has anyone else tried to handle something like this before?
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