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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