I have a single table polymorphism question. It looks like I can't
directly set the value of the polymorphic discriminator when creating
records; it is only set correctly when you instantiate objects of the
derived class. Is that true? Why, or what is wrong here?
Here is the scenario:
Jobs have Steps, Steps have Inputs of different types (say "SRC1" and
"SRC2")
Use declarative to set up these classes:
class Job(Base):
__tablename__ = 'job'
id = Column(Integer, primary_key=True, autoincrement=True)
title = Column(String(20))
jobnum = Column(Integer)
steps = relation('Step', backref=('jobs'))
def __repr__(s):
return '<Job> %s title=%s'%(s.id,s.title)
class Step(Base):
__tablename__ = 'step'
id = Column(Integer, primary_key=True, autoincrement=True)
name = Column(String(20))
job_id = Column(Integer, ForeignKey('job.id'))
inputs = relation('Input', backref=('step'))
def __repr__(s):
return '<Step> %s name=%s'%(s.id,s.name)
class Input(Base):
__tablename__ = 'input'
id = Column(Integer, primary_key=True, autoincrement=True)
kind = Column(String(10))
value = Column(String(30))
step_id = Column(Integer, ForeignKey('step.id'))
__mapper_args__ = {'polymorphic_on': kind}
def __repr__(s):
return '<Input> %s kind=%s value=%s'%(s.id,s.kind,s.value)
class SRC1(Input):
__mapper_args__ = {'polymorphic_identity': 'SRC1'}
src1file = Column(String(100))
class SRC2(Input):
__mapper_args__ = {'polymorphic_identity': 'SRC2'}
src2file = Column(String(100))
insert some data like this
j1 = Job(title='Job#201',jobnum=201)
step = Step(name='job201-step1')
j1.steps.append(step)
inp1 = SRC1(value='this one works')
step.inputs.append(inp1)
inp2 = Input(kind='SRC2',value='why is kind = NULL here')
step.inputs.append(inp2)
inp3 = Input(value='kind is null here also')
inp3.kind='SRC2'
step.inputs.append(inp3)
session.add(j1)
session.commit()
when i retrieve the data notice that the 2nd and 3rd records have no
kind value, even though I tried to explicitly set it.
for i in session.query(Input):
print i
<Input> 1 kind=SRC1 value=this one works
<Input> 2 kind=None value=why is kind = NULL here
<Input> 3 kind=None value=kind is null here also
So what happened with inp2 and inp3 where I tried to assign a value to
"kind"?
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