Relazioni SQLAlchemy su più tabelle

Question

Sto provando a creare una relazione che si estende su 3 tabelle ma non riesco a capire la sintassi.

Ho 3 tabelle TableA, TableB e TableC e il rapporto che sto cercando di modello è:

TableA.my_relationship = relationship(
    'TableC', 
    primaryjoin='and_(TableA.fk == TableB.pk, TableB.fk == TableC.pk)', 
    viewonly=True 
) 

in modo che su un'istanza di TableA posso fare instance_of_a.my_relationship per vedere la scheda TableC che è indirettamente associata con instance_of_a

Answer 1

from sqlalchemy import * 
from sqlalchemy.orm import * 
from sqlalchemy.ext.declarative import declarative_base 

Base = declarative_base() 

class A(Base): 
    __tablename__ = 'a' 

    id = Column(Integer, primary_key=True) 
    b_id = Column(Integer, ForeignKey('b.id')) 

    # method one - put everything into primaryjoin. 
    # will work for simple lazy loads but for eager loads the ORM 
    # will fail to build up the FROM to correctly include B 
    cs = relationship("C", 
       # C.id is "foreign" because there can be many C.ids for one A.id 
       # B.id is "remote", it sort of means "this is where the stuff 
       # starts that's not directly part of the A side" 
       primaryjoin="and_(A.b_id == remote(B.id), foreign(C.id) == B.c_id)", 
       viewonly=True) 

    # method two - split out the middle table into "secondary". 
    # note 'b' is the table name in metadata. 
    # this method will work better, as the ORM can also handle 
    # eager loading with this one. 
    c_via_secondary = relationship("C", secondary="b", 
         primaryjoin="A.b_id == B.id", secondaryjoin="C.id == B.c_id", 
         viewonly=True) 
class B(Base): 
    __tablename__ = 'b' 

    id = Column(Integer, primary_key=True) 
    c_id = Column(Integer, ForeignKey('c.id')) 

class C(Base): 
    __tablename__ = 'c' 
    id = Column(Integer, primary_key=True) 

e = create_engine("sqlite://", echo=True) 
Base.metadata.create_all(e) 

sess = Session(e) 

sess.add(C(id=1)) 
sess.flush() 
sess.add(B(id=1, c_id=1)) 
sess.flush() 
sess.add(A(b_id=1)) 
sess.flush() 

a1 = sess.query(A).first() 
print(a1.cs) 

print(a1.c_via_secondary)