On 08/04/2016 10:14 AM, Florian Rüchel wrote:
I have a relationship that depends on a query time variable to determine
the correct join. The use case is request-time localization in a web
application. When running the query during a request, I want to
determine the locale and only load the translation for the current
language for a given object. However, the primaryjoin condition callable
is evaluated at mapping time instead which only happens once instead of
on every request.
Here is a quick sample:
def get_myobj_primaryjoin():
return and_(MyObj.id == MyObjI18N.obj_id, request.locale ==
MyObjI18N.lang)
class MyObj(Base):
id = Column(Integer, primary_key=True)
_current_translation = relationship(MyObjI18N, uselist=False,
primaryjoin=get_myobj_primaryjoin, lazy='joined')
class MyObjI18N(Base):
obj_id = Column(ForeignKey(MyObj.id), primary_key=True)
lang = Column(String)
This should give a rough idea of the issue: request.locale changes at
query time, that is, if I do MyObj.query in two different requests, it
won't work, it will always take the first time it was called.
Note that I was previously using a with_transformation approach when
building the query but I wanted to remove the necessity to add that
every time a build a query and would have it much rather built implicitly.
Any ideas are highly appreciated, no argument I can pass to
"relationship" seems to help my use case.
we use a bound parameter for this and a recipe for getting a value in
there can be seen at
https://bitbucket.org/zzzeek/sqlalchemy/wiki/UsageRecipes/GlobalFilter .
In particular the lazyload case can only be affected using a custom
MapperOption as described near the bottom of that recipe.
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