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.
>
>
> models.EcRecipe.query.first().controller
>
>
> I get this error
>
> Traceback (most recent call last):
> File
> "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/code.py",
> line 90, in runcode
> exec(code, self.locals)
> File "<console>", line 1, in <module>
> File "/Users/cbarra/Projects/kompost/recipes/models.py", line 262, in
> controller
> deferred=True,
> File "<string>", line 2, in column_property
> File "<string>", line 2, in __init__
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/util/deprecations.py",
> line 130, in warned
> return fn(*args, **kwargs)
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/properties.py",
> line 134, in __init__
> self._orig_columns = [expression._labeled(c) for c in columns]
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/properties.py",
> line 134, in <listcomp>
> self._orig_columns = [expression._labeled(c) for c in columns]
> File "<string>", line 1, in <lambda>
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/sql/elements.py",
> line 4373, in _labeled
> return element.label(None)
> AttributeError: 'NoneType' object has no attribute 'label'
>
>
> while when I try to use `undefer` with
>
>
> models.EcRecipe.query.options(orm.undefer('controller'))
>
>
> the error is different
>
> Traceback (most recent call last):
> File
> "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/code.py",
> line 90, in runcode
> exec(code, self.locals)
> File "<console>", line 1, in <module>
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/query.py",
> line 1523, in options
> return self._options(False, *args)
> File "<string>", line 2, in _options
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/base.py",
> line 220, in generate
> fn(self, *args[1:], **kw)
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/query.py",
> line 1542, in _options
> opt.process_query(self)
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/strategy_options.py",
> line 168, in process_query
> self._process(query, True)
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/strategy_options.py",
> line 542, in _process
> raiseerr,
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/strategy_options.py",
> line 720, in _bind_loader
> raiseerr,
> File
> "/Users/cbarra/Projects/.venv/lib/python3.7/site-packages/sqlalchemy/orm/strategy_options.py",
> line 235, in _generate_path
> attr = found_property = attr.property
> AttributeError: 'property' object has no attribute 'property'
>
> I am bit puzzled between property, hybrid_property and column_property, what
> is the best way to have lazy attributes defined on a mixin?
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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