SQLAlchemy JOIN与邻接列表

Question

Consider the following model:

class A(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    bs = db.relationship('B', backref='A')

class B(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    cs = db.relationship('C', backref='A')

class C(db.Model):
    id = db.Column(db.Integer, primary_key=True)

Suppose I want to access the C ids given an object A. Is this the correct (and fastest) way to do this? Or should I go for a JOIN query or other SQL magic?

a = A.query.get(1)
c_ids = [c.id for b in a.bs for c in b.cs]

Answer 1

* EDIT * Changing C per A in the queries!

You can create a simple query to solve your issue as following:

q = session.query(C).join(B).join(A)
c_list = q.all()

c_list will contain every object matching the join condition. If you want to see the generated query, it's easy:

print(str(q))

SELECT c_table.c_id AS c_table_c_id, c_table.id AS c_table_id FROM c_table JOIN b_table ON b_table.id = c_table.c_id JOIN a_table ON a_table.id = b_table.a_id

* EDIT * This approach is more efficient than iterating over python objects as: You only have one single query that is resolved by the DB, if you iterate over your objects SQLAlchemy might need to fire some triggers to load the remaining objects from the database read loading relationships .

Next is the script I've used to replicate your case:

from sqlalchemy import Column, create_engine, ForeignKey
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.types import Integer
from sqlalchemy.ext.declarative import declarative_base

model = declarative_base()

class A(model):
    __tablename__ = 'a_table'
    id = Column(Integer, primary_key=True)
    bs = relationship('B', backref='A')

class B(model):
    __tablename__ = 'b_table'
    a_id = Column(Integer, ForeignKey('a_table.id'))
    id = Column(Integer, primary_key=True)
    cs = relationship('C', backref='B')

class C(model):
    __tablename__ = 'c_table'
    c_id = Column(Integer, ForeignKey('b_table.id'))
    id = Column(Integer, primary_key=True)


engine = create_engine('postgresql+psycopg2://playground:playground@localhost:5432/playground')
session_factory = sessionmaker(bind=engine)
session = session_factory()

model.metadata.create_all(engine)
a = A()
b = B(A=a)
c = C(B=b)
session.add(a)
session.commit()

q = session.query(C).filter(C.id == 1)
c = q.first()
print(c.B.A.id)

q = session.query(A).filter(A.id == 1)
a = q.first()
c_id = [c.id for a in a.bs for c in b.cs]

q = session.query(C).join(B).join(A)
c_list = q.all()

print(str(q))