On Thu, Nov 28, 2019, at 4:45 AM, Carson Ip wrote:
> Hi Mike,
>
> I have having issues letting a subclass load a parent's relationship that
> configured lazy="joined".
>
> Let's say Engineer is a subclass of Person, and Person stores "company_id"
> and has a relationship called "company" that is lazy="joined".
>
> Although I don't have a minimal reproducible example now, but in my setup,
> let's say:
> engineer = session.query(Engineer).get(1) # emits 1 query
> engineer.company # emits another query
>
> I suspect that the subclass does not respect parent class' relationship
> lazy/eager loading options.
nope, it does, no issue there so you need an MCVE
>
> Also I bump into this issue when identity map remembers the id's polymorphic
> type. Then when I query on the base class, it emits the the subclass query
> and does not load the parent class' relationship with eager load options,
> which is the problem I mentioned.
> engineer = session.query(Engineer).get(1)
> session.commit()
> person = session.query(Person).get(1) # emits 1 query
> person.company # emits another query
>
>
>
> Is this by design?
nope, those are the same object, if it is loading two different objects for the
same id, need an MCVE, sounds like your polymorphic_discriminator is not set
correctly. the query for .company is expected there because it is expired by
the commit() previously. the refresh which occurs on the get() does not use a
joined eager load until 1.4.
>
> If so, is there any way to make it respect parent class' relationship "lazy"
> option?
it does, need an MCVE (see the theme? :) )
>
> I am not 100% certain this is reproducible in a simple example. There may be
> problems in my setup that would cause this. Please let me know if this is the
> case.
so then you start with a simple example and then slowly modify your real
program one line at a time until it looks like your simple example, each time
testing to see what causes the problem.
here's all your test cases
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
Base = declarative_base()
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
employees = relationship("Person", back_populates="company")
class Person(Base):
__tablename__ = "people"
id = Column(Integer, primary_key=True)
discriminator = Column("type", String(50))
company_id = Column(ForeignKey("company.id"))
__mapper_args__ = {
"polymorphic_on": discriminator,
"polymorphic_identity": "person",
}
company = relationship(
"Company", lazy="joined", back_populates="employees"
)
class Engineer(Person):
__tablename__ = 'engineer'
id = Column(Integer, ForeignKey('people.id'), primary_key=True)
primary_language = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
s = Session(e)
s.add(Company(employees=[Engineer(primary_language="java")]))
s.commit()
s.close()
e1 = s.query(Engineer).get(1)
# close so we assert no sql in the next step
s.close()
# no query
assert e1.company is not None
engineer = s.query(Engineer).get(1)
s.commit()
person = s.query(Person).get(1) # emits 1 query
# same object
assert engineer is person
# s.close() # you can do this if you are on 1.4. in 1.3, the .company attr is
not loaded
# emits a query in 1.3 because the attribute was expired by the commit
person.company
>
> Thanks in advance.
>
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> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
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> To post example code, please provide an MCVE: Minimal, Complete, and
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