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 <javascript:>> > 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. > > -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/groups/opt_out.