I am trying to create a relationship from one table to another, which
involves two intermediary tables. I *think* I need to use the secondaryjoin
+ secondary arguments to relationship(). But after studying the
documentation for a long time, I can't get my head around how these
arguments are supposed to work.
Here is my schema:
class Person(Base):
__tablename__ = "person"
id = Column(Integer, primary_key=True)
class PersonInstance(Base):
__tablename__ = "person_instance"
id = Column(Integer, primary_key=True)
person_id = Column(Integer, ForeignKey("person.id"))
class Photo(Base):
__tablename__ = "photo"
id = Column(Integer, primary_key=True)
class PhotoInstance(Base):
__tablename__ = "photo_instance"
id = Column(Integer, primary_key=True)
photo_id = Column(Integer, ForeignKey("photo.id"))
person_instance_id = Column(Integer, ForeignKey("person_instance.id"))
I want to create a one-to-many relationship *Person.photos* which goes from
Person -> Photo. A Person is one-to-many with PersonInstance, and a Photo
is one-to-many with PhotoInstance objects. The connection from a Person to
a Photo exists between PersonInstance and PhotoInstance, via the
PhotoInstance.person_instance_id foreign key.
First I tried using only primaryjoin:
photos = relationship(
"Photo",
primaryjoin=(
"and_(Person.id==PersonInstance.person_id, "
"PersonInstance.id==PhotoInstance.person_instance_id, "
"PhotoInstance.photo_id==Photo.id)"
)
)
I got an error saying it couldn't find the necessary foreign keys to
compute the join.
So now I'm messing with secondary + secondaryjoin, but it's really trial &
error as I don't know how these arguments are supposed to work in my case.
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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