On Aug 2, 2010, at 12:19 PM, Robert Sudwarts wrote:
> Hi,
>
> I'm having trouble understanding the correct syntax to be used with
> Declarative and a column property.
> The select statement I'm using is: select([func.substr(my_table.c.my_string,
> 2, 3)]).label(my_substr'), deferred=True
> And (as per the docs using the expanded syntax, this works as expected.
>
> With the Declarative syntax, I've tried:
>
> from sqlalchemy.util import classproperty
> from sqlalchemy.orm ..., column_property
>
> class MySubstr(object):
> @classproperty
> def my_substr(cls):
> return column_property(
>
> select([func.substr(cls.my_str, 2, 3)]).label('my_substr')
> )
>
> class MyTable(Base, MySubstr):
> .....
>
> and then expect to be able to call a record in MyTable and return its
> "my_substr", however, all I'm getting is a representation of the object ...
> eg <sqlalchemy.orm.properties.ColumnProperty object at 0xa4951cc> and no
> apparent way of seeing its value.
I see nothing wrong with the example, the only potential glitch is that
"my_str" needs to have a name assigned up front. You didn't illustrate that
part here, so FYI it would be extremely helpful if you could attach fully
working examples. In this case the example is:
from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.util import classproperty
from sqlalchemy.orm import column_property, sessionmaker
Base = declarative_base()
engine = create_engine('sqlite://', echo=True)
class MySubstr(object):
@classproperty
def my_substr(cls):
return column_property(
select([func.substr(cls.my_str, 2,
3)]).label('my_substr')
)
class MyTable(Base, MySubstr):
__tablename__ = 'foo'
id = Column(Integer, primary_key=True)
my_str = Column('my_str', String)
Base.metadata.create_all(engine)
sess = sessionmaker(engine)()
m1 = MyTable(my_str='some string')
sess.add(m1)
sess.commit()
print m1.my_substr
and it works fine.
>
> Should the @classproperty sit within the "MyTable(Base)" itself (ie is there
> a need for the separate object --
@classproperty is typically specific to the mixin use case, which is why its
only discussed in the "mixins" section of the declarative documentation. It's
a useful construct in other situations but you certainly don't need it if you
aren't using mixins.
> and have I been confused by the section on 'mixins' )?
I'm not sure why you're choosing to use a mixin here, as your column_property
appears to be pretty specific to the MyTable class. if the documentation led
you to believe that you needed one, I am extremely curious why that is the case
as mixins are absolutely optional and that would be a serious documentation
bug. Without the mixin, it is:
class MyTable(Base):
__tablename__ = 'foo'
id = Column(Integer, primary_key=True)
my_str = Column('my_str', String)
my_substr = column_property(
select([func.substr(my_str, 2,
3)]).label('my_substr')
)
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