If you can't establish the event at class declaration time, then the
@event.listens_for/event.listen() paradigm (see
http://docs.sqlalchemy.org/en/latest/core/event.html for background) can be
applied to the class-bound attribute (which here is "Positive.value") at any
time. "Positive.value" is an attribute generated by the mapping which is a
result of using the declarative base.
On Apr 15, 2012, at 1:10 PM, lars van gemerden wrote:
> OK, this helps, so how do i do this if i do not know the name of the
> attribute to be checked in advance(the "value" in
> @validates("value") )?
>
>
>
> On Apr 15, 6:52 pm, Michael Bayer <[email protected]> wrote:
>> using type() is equivalent to using a class declaration. The end result is
>> the same, as are the mechanics of what goes on both from a Python as well as
>> a SQLAlchemy perspective. So this works:
>>
>> Positive = type("Positive", (Base,), dict(__tablename__ =
>> "positives", value = Column(Integer)))
>>
>> @event.listens_for(Positive.value, "set")
>> def checkvalue(target, value, oldvalue, initiator)
>> assert value > 0
>>
>> and also, since type() is equivalent to a class declaration, you can still
>> use @validates, if you pass the function into the class dictionary, so that
>> it is part of the class before declarative sends it off to mapper():
>>
>> @validates("value")
>> def checkvalue(self, name, value):
>> assert value > 0
>> return value
>>
>> Positive = type("Positive", (Base,), dict(__tablename__ =
>> "positives", value = Column(Integer), checkvalue=checkvalue))
>>
>> On Apr 15, 2012, at 12:37 PM, lars van gemerden wrote:
>>
>>
>>
>>
>>
>>
>>
>>> I don't know what "@validates hangs a marker of the method that
>>> mapper() uses when it instruments the class" means. I guess my
>>> question now becomes: How do I add the event.listens_for descriptor to
>>> the class, since i do not have a class declaration in the traditional
>>> sense?
>>
>>> On Apr 15, 4:29 pm, Michael Bayer <[email protected]> wrote:
>>>> @validates hangs a marker of the method that mapper() uses when it
>>>> instruments the class, so if the class is already mapped then that train
>>>> has left the station. Taking a cab instead, you can just add the
>>>> attribute event directly:
>>
>>>> @event.listens_for(Positive.value, "set")
>>>> def checkvalue(target, value, oldvalue, initiator)
>>>> assert value > 0
>>
>>>> if you want to return a new, mutated value then add retval=True to
>>>> listens_for().
>>
>>>> On Apr 15, 2012, at 8:22 AM, lars van gemerden wrote:
>>
>>>>> Hi,
>>
>>>>> I need a way to dynamically add a validates method to a already
>>>>> created sqla class.
>>
>>>>> In a normal class declaration you can add a validator by:
>>
>>>>> class Positive(Base):
>>>>> __tablename__ = "positives"
>>>>> value = Column(Integer)
>>
>>>>> def checkvalue(self, name, value):
>>>>> assert value > 0
>>>>> return value
>>>>> validates("value")(checkvalue)
>>
>>>>> However if you get the class dynamically:
>>
>>>>> Positive = type("Positive", (Base,), dict(__tablename__ =
>>>>> "positives", value = Column(Integer)))
>>
>>>>> I can't figure out how to add the validator, either in the type() call
>>>>> or afterwards.
>>
>>>>> Cheers, Lars
>>
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>>
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