On 9/25/15 8:22 AM, Adrian wrote:
For some methods/properties on a model it might be useful to memoize
its result.
There are some common decoratos such as cached_property from werkzeug
which simply add the
value to the object's __dict__ after retrieving it the first time
(thus not calling the property's getter again).
Or you might end up using custom memoization code that stores the
value somewhere on the object.
Anyway, the problem with all those things is that I'm likely to cache
things too long in case they depend
on a column/relationship value since expiring the object won't expire
my cached data.
Is there any place where I could store custom data associated with an
instance of a mapped object that
is cleared when the object is expired?
one difficulty with that is that the "expired' state isn't
all-or-nothing in some cases; individual attributes can be expired which
may or may not be part of your attribute's state.
for that reason you want to write an expire handler using the expire
event:
http://docs.sqlalchemy.org/en/rel_1_0/orm/events.html#sqlalchemy.orm.events.InstanceEvents.expire
which will pop your __dict__-cached values when this is intercepted.
In the vast majority of cases this occurs for all attributes so it would
be feasible to disregard checking the attrs collection.
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