Please see my comments below.
Regards,
Rob
On 3 August 2010 14:05, Michael Bayer <mike...@zzzcomputing.com> wrote:
>
> On Aug 3, 2010, at 7:47 AM, Robert Sudwarts wrote:
>
> Hi Michael,
>
> I'm guilty of two over-simplifications in the code used in my example...
>
> Firstly, the substring property/column (with which I had a problem) applies
> to a subclass (using "joined table inheritance") and the variable to which
> it applies is specified in its parent class. Hence, I started by assuming
> that I'd need something along the lines of:
>
> class Parent(Base):
> my_string
>
> class Child(Parent):
> ....
> @classproperty/@property/ some other decorator
> def my_substr(cls):
> return column_property ( .... cls.my_string .... )
>
> However, trying this (ie the classproperty/column_property within Child),
> returned <sqlalchemy.orm.properties.ColumnProperty object ...> (the error
> I referred to in my original question)
> I'm confused about why I'm not able to get this (or some variation of it)
> to work. Is this indeed possible?
>
>
>
> Just yesterday, in response to this, I added a line of code such that if
> you specifically use @classproperty on the declarative class (i.e. not a
> mixin), declarative will call it to figure out what it returns and configure
> it normally- previously, if the attribute were not directly a
> MapperProperty, it would be ignored. However, I couldn't think of any use
> case that this is actually useful for. I thought that, it might be a way to
> establish a column-oriented property "late", so that you could get around
> imports not being available. At the moment, it's not useful for that,
> because declarative calls the attribute as soon as the class is created, and
> whatever imports that werent available to the class declaration are still
> not present.
>
Ah! Right!! I hadn't understood that ... I assumed (wrongly) that I'd have
to 'instantitate' 'cls' somehow using a decorator+function: def blah(cls):
...
I didn't realise that I could simply use "Parent.my_str"
>
> So what are you gaining above, if it were working in 0.6.3, by using
> @classproperty instaed of "my_substr = column_property()" ? Anywhere you
> reference "cls" in your function, you'd just reference whatever is local, or
> part of Parent.
>
>
> Looking at the documentation (as a consequence of the error), my I was
> drawn to the syntax used in the section on 'mixins' (ie, the column_property
> defined externally).
>
> My second over-simplification regards the the function itself. The
> function I'm trying to use [which I'm aware doesn't translate easily to
> sqlite], is:
> func.substr(cls.my_str, func.instr(cls.my_str, " ")+1, 2)
>
> Now, thanks to your response, I now actually have the following *working*
> (using mysql):
>
> class MySubStr(object):
> @classproperty
> def my_substr(cls):
> return column_property(
> func.substr(cls.my_string,
> func.instr(cls.my_string, " ")+1, 2)
> )
>
> However, replacing:
> func.substr(cls.my_string, func.instr(cls.my_string, " ")+1, 2)
> with (in order to attach a label to the column):
> select([func.substr(cls.my_string, func.instr(cls.my_string, " ")+1,
> 2)]).label('exch_code')
>
> Gives me -- AttributeError: 'NoneType' object has no attribute '_keymap'
> (full traceback below).
>
>
> That's a really weird error. It suggests that the result is not one that
> returns any rows, not like its a SELECT with zero rows, rather, it is
> perceived by SQLite as something like an INSERT that isn't meant to return
> any rows, and there's no cursor.description. The ResultProxy is then
> failing ungracefully which is surprising (looking at the current source it
> appears we need to cover this situation). It would be interesting to try
> out the raw SQL to see what is produced, since the statement is definitely a
> SELECT - pysqlite should raise an error if it doesn't like the SQL, or give
> us a cursor.description.
>
Just to be clear -- this error/traceback was produced with MySQL as the
database....
>
> Anyway, your column_property() doesn't need the select() here, you can just
> call .label() on the func.substr() result, but also, you don't really need
> the label() in here either (unless I'm missing something? )
>
> Again ... I hadn't realised that I could apply .label() to the directly to
'func' ... I've changed this and it works.
>
> Apologies for my earlier over-simplification (and my late response!). And
> I hope that what I've responded with is clear!!
>
> Regards,
> Rob
>
>
>
>
>
> -------------------------------------------------------------------------------------------------------------------------
> /home/robertsudwarts/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.6.3-py2.6.egg/sqlalchemy/orm/query.pyc
> in one(self)
> 1520
> 1521 """
> -> 1522 ret = list(self)
> 1523
> 1524 l = len(ret)
>
> /home/robertsudwarts/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.6.3-py2.6.egg/sqlalchemy/orm/query.pyc
> in instances(self, cursor, _Query__context)
> 1667 break
> 1668 else:
> -> 1669 fetch = cursor.fetchall()
> 1670
> 1671 if custom_rows:
>
> /home/robertsudwarts/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.6.3-py2.6.egg/sqlalchemy/engine/base.pyc
> in fetchall(self)
> 2381
> 2382 try:
> -> 2383 l = self.process_rows(self._fetchall_impl())
> 2384 self.close()
> 2385 return l
>
> /home/robertsudwarts/tg2env/lib/python2.6/site-packages/SQLAlchemy-0.6.3-py2.6.egg/sqlalchemy/engine/base.pyc
> in process_rows(self, rows)
> 2364 process_row = self._process_row
> 2365 metadata = self._metadata
> -> 2366 keymap = metadata._keymap
> 2367 processors = metadata._processors
> 2368 if self._echo:
>
> AttributeError: 'NoneType' object has no attribute '_keymap'
>
>
>
>
>
>
>
>
>
> On 2 August 2010 19:42, Michael Bayer <mike...@zzzcomputing.com> wrote:
>
>>
>> On Aug 2, 2010, at 1:40 PM, Michael Bayer wrote:
>>
>> >
>> > 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.
>> >
>>
>> oh and also, you don't need the select() here either:
>>
>> class MyTable(Base):
>> __tablename__ = 'foo'
>> id = Column(Integer, primary_key=True)
>> my_str = Column('my_str', String)
>> my_substr = column_property(func.substr(cls.my_str, 2, 3))
>>
>>
>>
>>
>> > 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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