複数のテーブルにわたるSQLAlchemy関係

Question

3つのテーブルにわたる関係を作成しようとしていますが、構文を理解できません。

私は3つのテーブルTableA、TableBとTableCを持っていると私はモデルにしようとしている関係は次のとおりです。TableAのインスタンス上で、私は間接的に関連するのですTableCレコードを取得するためにinstance_of_a.my_relationshipを行うことができるように

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

with 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)