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
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?
--
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The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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