@gdelta.expression is a typo. Should be @delta.expression.
On Wednesday, February 12, 2014 11:53:05 AM UTC-7, Eric Atkin wrote:
>
> Yeah sorry I missed that. conversion is an attribute on Measured_Source.
> So the intent is that a Production_Load is a Load with its own additional
> attributes over Load as well as a constraint that its source is a
> Measured_Source which has its own attribute extensions over Source. One of
> the goals here is to add that constraint enforcement. I was able to make it
> work with the following hybrid_method and hybrid_method.expression, but the
> isinstance(Production_Load.source, Measured_Source) enforcement is missing.
> Eric
>
> {{{
> class Production_Load(Load):
> __tablename__ = 'production_load'
> __mapper_args__ = { 'polymorphic_identity':'production_load' }
>
> id = Column(Integer, ForeignKey('load.id'), primary_key=True)
> top = Column(Numeric, nullable=False)
> bottom = Column(Numeric, nullable=False)
>
> @hybrid_method
> def delta(self):
> return (self.top-self.bottom)*self.source.conversion if
> self.source else None
>
> @gdelta.expression
> def delta(self):
> # not sure about the performance here
> return
> (self.top-self.bottom)*select([Measured_Source.conversion]).where(Measured_Source.id==self.source_id).label('delta')
> }}}
>
> On Wednesday, February 12, 2014 7:17:08 AM UTC-7, Michael Bayer wrote:
>>
>>
>> On Feb 11, 2014, at 9:38 PM, Eric Atkin <[email protected]> wrote:
>>
>> > Hi,
>> > I want to override a relationship in a subclass to relate to a subclass
>> of the base attributes' related class. Perhaps an example of how I thought
>> it should work:
>> >
>> > {{{
>> > class Load(Base):
>> > __tablename__ = 'load'
>> > __mapper_args__ = {
>> > 'polymorphic_identity':'load',
>> > 'polymorphic_on':'polymorphic_type',
>> > }
>> > id = Column(Integer, primary_key=True)
>> > polymorphic_type = Column(Text, nullable=False)
>> > source_id = Column(Integer, ForeignKey('source.id'))
>> > source = relationship('Source')
>> >
>> > class Production_Load(Load):
>> > __tablename__ = 'production_load'
>> > __mapper_args__ = { 'polymorphic_identity':'production_load' }
>> > id = Column(Integer, ForeignKey('load.id'), primary_key=True)
>> > source_id = Column(Integer, ForeignKey('measured_source.id'))
>> > source = relationship('Measured_Source')
>> >
>> > class Source(Base):
>> > __tablename__ = 'source'
>> > __mapper_args__ = {
>> > 'polymorphic_identity':'source',
>> > 'polymorphic_on':'polymorphic_type',
>> > }
>> > id = Column(Integer, primary_key=True)
>> > polymorphic_type = Column(Text, nullable=False)
>> >
>> > class Measured_Source(Source):
>> > __tablename__ = 'measured_source'
>> > __mapper_args__ = { 'polymorphic_identity':'measured_source' }
>> >
>> > id = Column(Integer, ForeignKey('source.id'), primary_key=True)
>> > }}}
>> >
>> > As you can see, we have Load.source -> Source and I want
>> Production_Load.source -> Measured_Source, but when I import the models, I
>> get the following warning:
>> >
>> >
>> {...}/env/lib/python2.7/site-packages/sqlalchemy/orm/properties.py:1028:
>> SAWarning: Warning: relationship 'source' on mapper
>> 'Mapper|Production_Load|production_load' supersedes the same relationship
>> on inherited mapper 'Mapper|Load|load'; this can cause dependency issues
>> during flush
>> >
>> > and when I try to use Production_Load.source (class level attr) in a
>> query, I get the following error:
>> >
>> > AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator'
>> object associated with Production_Load.source has an attribute 'conversion'
>> >
>> > Is such a thing possible, even with a re-factor of the models?
>>
>> “conversion” sounds like an attribute name on your end, but generally
>> being able to supersede a relationship like that when the inheritance is
>> not “concrete” is not supported. you’d need to name it to something else.
>>
>>
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