Il giorno mercoledì 5 giugno 2019 18:17:48 UTC+2, Mike Bayer ha scritto:
>
>
>
> On Wed, Jun 5, 2019, at 12:02 PM, Christian Barra wrote:
>
> Hi, I am trying to understand what the best approach is to have lazy
> attributes defined on a mixin and then used them on the subclasses.
>
> Ideally I'd would like to defer the load of that attribute (controller)
> and use `undefer` when I know that that attribute is needed, to execute
> only one query.
>
> Below there's the code I am trying to use, `recipe_set` is a backref, same
> for `unit`, Controller is another model.
>
>
> class BaseRecipeMixin(APIResourceMixin):
> _key = None
>
> @declared_attr
> def recipe_set_id(self):
> return db.Column(db.ForeignKey('recipe_set.id'), unique=True,
> nullable=False)
>
> @property
> def controller(self):
> return column_property(
> Controller.query.filter(
> Controller.unit == self.recipe_set.unit,
> Controller.is_master.is_(True),
> Controller.is_deleted.is_(False),
> ).one_or_none(),
> deferred=True,
> )
>
>
> class EcRecipe(DosingRecipeMixin, BaseRecipeMixin, UpdateMixin, db.Model):
> id = db.Column(db.Integer, primary_key=True)
> pump_time = db.Column(db.Float, nullable=False, default=8)
>
>
> But when I try to run this
>
>
>
> when you declare mapped attributes on a mixin you have to use
> @declared_attr:
>
>
> https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/mixins.html#mixing-in-deferred-column-property-and-other-mapperproperty-classes
>
> The column_property() object needs to be made part of the mapping and
> declarative uses this decorator to know it has to do that.
>
>
>
Thanks Mike, I'll try to use @declared_attr and go through the link.
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