将 SQLAlchemy 与复合主键和外键一起使用

Question

I am trying to create a relationship between two tables using declarative SQLAlchemy (v1.1.5) where the tables have composite primary and foreign keys.

This is essentially a classic One-To-Many relationship between two tables, where Resource defines the parent, and each resource has multiple segments, defined in the Resourcesegment table. The twist is that I've added a VersionID column to both tables, so I can use the database with multiple versions of the data. Here's the simplified code:

from sqlalchemy import Column, String, Integer, ForeignKeyConstraint
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

Base = declarative_base()

class Resource(Base):
    __tablename__ = 'resource'
    VersionID = Column(String, primary_key=True)
    ResourceID = Column(String, primary_key=True)
    ResourceProperty = Column(String)
    segments = relationship("Resourcesegment", back_populates="resource")

class Resourcesegment(Base):
    __tablename__ = 'resourcesegment'
    VersionID = Column(String, primary_key=True)
    ResourceID = Column(String, primary_key=True)
    Segment_Type = Column(String, primary_key=True)
    Segment_Number = Column(Integer, primary_key=True)
    Segment_Property = Column(String)
    resource = relationship("Resource", foreign_keys=[VersionID, ResourceID], back_populates="segments")

    __table_args__ = (ForeignKeyConstraint([VersionID, ResourceID], [Resource.VersionID, Resource.ResourceID]),)

The error message I am getting is:

InvalidRequestError: Mapper 'Mapper|Resourcesegment|resourcesegment' has no property 'resource'

I've tried many different configurations for relationships and foreign keys with no luck. From what I've understood in the documentation and various Stackoverflow examples, this is my best guess at what should work. Clearly I'm missing something.

Thanks for any help / suggestions / comments!

Answer 1

Assuming you're trying to use the segment PKs as FKs to Resource, try to define the FKs on the PK attribute definitions, eg along the lines of:

class Resourcesegment(Base):
    __tablename__ = 'resourcesegment'
    VersionID = Column(String, ForeignKey(Resource.VersionID), primary_key=True)
    ResourceID = Column(String, ForeignKey(Resource.ResourceID), primary_key=True)
    Segment_Type = Column(String, primary_key=True)
    Segment_Number = Column(Integer, primary_key=True)
    Segment_Property = Column(String)
    resource = relationship("Resource", back_populates="segments")

Answer 2

So I am answering my own question! It turns out that the original code I posted above is correct. The problem I was experiencing was due to having a much more complicated set of tables in the actual model I am developing. One of the other tables had a problem, which ended up throwing an error making it look like the problem was with the above tables. When I stripped everything away, as a result of creating this SO post, I got my simplified model to work, and was able to isolate the problem to another part of the bigger model.

One caveat: The error messages coming out of SQLAlchemy can be quite confusing, and not necessarily able to pinpoint the actual problem, without doing some extra debug work.