On Dec 13, 2009, at 12:09 PM, Daishy wrote:
> Hi together,
>
> I'm sorry if this is a rather stupid question, but i havent found a
> solution yet :/
> I have a few models build with the DeclarativeBase-Class. Now each of
> these models has a few columns that they have in common (created_by,
> created_at, updated_by, updated_at). Rather than putting the code i
> need for these columns and the columns itself in each model, i wanted
> to put it into a seperate class and inherit from it (or include it).
> But if i query ModelA i just get '(no name)' if i print the created_X
> columns. Is there any way to extract columns each model should have
> into a seperat class?
The Column objects you declare within declarative become members of the
underlying Table object, so its not as simple as just having those members
present on a mixin - what would really be needed would be some sort of copying
of each column object when declarative comes across them.
The mechanism of declarative is also to look at the "dict" of only the endpoint
class itself when picking up the attributes to add to the Table/mapper, so at
the moment any mixin would be ignored in any case.
There is an alternative approach to the "place these columns on every class"
problem, which involves making a subclass of the metaclass that adds the
desired columns when a new class is created. I don't prefer this method since
we're usually talking about just a few columns, the metaclass itself is
slightly awkward, and at the end of the day I do like to see the columns listed
out on each class (hence "declarative"...).
What I usually do is to reduce the overhead by creating callables for the
desired columns (usually the same created_at etc. columns you're using here):
def created_at():
return Column(DateTime, default=func.now)
so you only need to add a little bit to each class:
class Foo(Base):
...
created_at = created_at()
updated_at = updated_at()
The "metaclass subclass" approach is somewhere in the list archives if you want
to search for it.
Also, it wouldn't be impossible for declarative to someday be enhanced to
accept "mixins", which probably would subclass a specific class to mark them,
and would be allowed to declare a limited set of "copyable" attributes, namely
Column objects and perhaps deferred()s also. I haven't tried working out
such a feature as of yet, however.
>
>
> class Data:
> created_by = Column(Integer)
> created_at = Column(DateTime)
>
>
> class ModelA(DeclarativeBase, Data):
> id = Column(Integer, primary_key=True)
> somethingelse = Column(Integer)
>
>
> Thanks very much!
> Daishy
>
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