If I understand the problem correctly your best shot would be using
sqlalchemy magical `hybrid_property` , hybrid_method, etc.
here:
http://docs.sqlalchemy.org/ru/latest/orm/extensions/hybrid.html
On Monday, March 18, 2013 9:20:15 PM UTC+2, millerdev wrote:
Hi,
Using declarative here, and I'm trying to create a column_property with a
correlated subquery that returns a count of records with a matching value
in some other column. Here's what I've tried. Option 1 is the best, option
2 is ugly but second best, option 3 is not a good option since there are
many other classes involved and the place where I'd need to put that code
is far away from where it logically belongs.
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import *
from sqlalchemy.ext.declarative import declared_attr
Base = declarative_base()
option = 1
class Foo(Base):
__tablename__ = 'foo'
id = Column(Integer, primary_key=True)
bar_id = Column(Integer, ForeignKey(bar.id))
name = Column(String)
if option == 1:
# does not work (see first traceback below)
@declared_attr
def name_count(cls):
clx = aliased(cls)
return column_property(
select(func.count([clx.id]))
.where(clx.name == cls.name)
.correlate(cls.__table__))
if option == 2:
# does not work (see second traceback below)
_foo = aliased(Foo)
Foo.name_count = column_property(
select([func.count(_foo.id)])
.where(_foo.name == Foo.name)
.correlate(Foo.__table__))
class Bar(Base):
__tablename__ = 'bar'
id = Column(Integer, primary_key=True)
name = Column(String)
if option == 3:
# works, but really not where I want to put this code
_foo = aliased(Foo)
Foo.name_count = column_property(
select([func.count(_foo.id)])
.where(_foo.name == Foo.name)
.correlate(Foo.__table__))
Option 1 traceback:
Traceback (most recent call last):
File temp/example.py, line 8, in module
class Foo(Base):
File .../python2.7/site-packages/sqlalchemy/ext/declarative.py, line
1348, in __init__
_as_declarative(cls, classname, cls.__dict__)
File .../python2.7/site-packages/sqlalchemy/ext/declarative.py, line
1181, in _as_declarative
value = getattr(cls, k)
File .../python2.7/site-packages/sqlalchemy/ext/declarative.py, line
1554, in __get__
return desc.fget(cls)
File temp/example.py, line 15, in name_count
clx = aliased(cls)
File .../python2.7/site-packages/sqlalchemy/orm/util.py, line 385, in
aliased
return AliasedClass(element, alias=alias, name=name,
adapt_on_names=adapt_on_names)
File .../python2.7/site-packages/sqlalchemy/orm/util.py, line 298, in
__init__
self.__mapper = _class_to_mapper(cls)
File .../python2.7/site-packages/sqlalchemy/orm/util.py, line 673, in
_class_to_mapper
raise exc.UnmappedClassError(class_or_mapper)
sqlalchemy.orm.exc.UnmappedClassError: Class '__main__.Foo' is not mapped
Option 2 traceback:
Traceback (most recent call last):
File temp/example.py, line 16, in module
select([func.count(_foo.id)])
File .../python2.7/site-packages/sqlalchemy/sql/expression.py, line
1229, in __call__
return func(*c, **o)
File .../python2.7/site-packages/sqlalchemy/sql/functions.py, line 16,
in __call__
args = [_literal_as_binds(c) for c in args]
File .../python2.7/site-packages/sqlalchemy/sql/expression.py, line
1440, in _literal_as_binds
return element.__clause_element__()
File .../python2.7/site-packages/sqlalchemy/orm/attributes.py, line
117, in __clause_element__
return self.comparator.__clause_element__()
File .../python2.7/site-packages/sqlalchemy/util/langhelpers.py, line
506, in oneshot
result = self.fget(obj, *args, **kw)
File .../python2.7/site-packages/sqlalchemy/orm/properties.py, line
156, in __clause_element__
return self.adapter(self.prop.columns[0])
File .../python2.7/site-packages/sqlalchemy/orm/util.py, line 334, in
__adapt_element
return self.__adapter.traverse(elem).\
File .../python2.7/site-packages/sqlalchemy/sql/visitors.py, line 185,
in traverse
return replacement_traverse(obj, self.__traverse_options__, replace)
File .../python2.7/site-packages/sqlalchemy/sql/visitors.py, line 281,
in replacement_traverse
obj = clone(obj, **opts)
File .../python2.7/site-packages/sqlalchemy/sql/visitors.py, line 270,
in clone
newelem = replace(elem)
File .../python2.7/site-packages/sqlalchemy/sql/visitors.py, line 182,
in replace
e = v.replace(elem)
File .../python2.7/site-packages/sqlalchemy/sql/util.py, line 720, in
replace
return self._corresponding_column(col, True)
File .../python2.7/site-packages/sqlalchemy/sql/util.py, line 695, in
