@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 <eat...@certusllc.us> 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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