Combining relationships

Question

I have a model Zone, a model Entity and a model Transit. Transit is minimally defined as:

class Entity(db.Model):
    __tablename__ = 'entities'
    id = db.Column(db.Integer, primary_key=True)
    contained_by = db.Column(db.Integer, db.ForeignKey('entities.id'))
    contains = db.relationship('Entity', backref='container')
    discriminator = db.Column('type', db.String(50))
    __mapper_args__ = {'polymorphic_on': discriminator}

class Zone(Entity):
    __mapper_args__ = {'polymorphic_identity': 'zones'}
    routes = db.relationship('Transit')
    (stuff goes here)

class Transit(db.Model):
    __tablename__ = "transits"
    start = db.Column(db.Integer, db.ForeignKey('zones.id'))
    end = db.Column(db.Integer, db.ForeignKey('zones.id'))

Zone also has a couple of bits about distance and how defensible it is, but that is irrelevant for this.

• First off, due to the fact that Zone is subclassed from Entity using single-table inheritance can I reference zones.id?
• Secondly, will the Zone.routes property merge Transit.start and Transit.end?

Answer 1

1. no, you need to use the table name, which in your case (Single-table inheritance) is entities
2. no, these will not be merged. You can create two relationships, and have a (hybrid) property which would combine both, but this will only be for reading purposes, as when you would like to modify this property (for example, add Transits), you would still need to specify both sides (start and end).
3. I am not sure I understand the question here

update-1: as requested in comment, Concrete-Table inheritance code below:

class Zone(Entity):
    __mapper_args__ = {'polymorphic_identity': 'zones'}
    __tablename__ = "zones"
    id          = Column(Integer, ForeignKey('entities.id'), primary_key=True)
    @property
    def transits(self):
        return self.transits_from_here + self.transits_to_here

class Transit(Base):
    __tablename__ = "transits"
    id          = Column(Integer, primary_key=True)
    start = Column(Integer, ForeignKey('zones.id'))
    end = Column(Integer, ForeignKey('zones.id'))

    s_zone = relationship(Zone, primaryjoin=(start==Zone.id), backref="transits_from_here")
    e_zone = relationship(Zone, primaryjoin=(end==Zone.id), backref="transits_to_here")