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association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)
class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Child",
                    secondary="association",
                    backref="parents")
class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)

parents = session.query(Parent).filter(Parent.children.any(id=2))

from sqlalchemy import create_engine, Column, Integer, Table, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
 
Base = declarative_base()
 
association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)
 
class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", secondary=association_table, backref="parent")
 
    def __repr__(self):
        return '<Parent(id={})>'.format(self.id)
 
class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    def __repr__(self):
        return '<Child(id={})>'.format(self.id)
 
 
engine = create_engine('mysql://{login}:{password}@{server}:{port}/{db}'.format(
            login='test',
            password='test',
            server='localhost',
            port=3306,
            db='test',
        ))
session = sessionmaker(bind=engine)()
 
# -------------------------------------------------------------------------------
 
p1 = Parent(id=1)
p2 = Parent(id=2)
 
c1 = Child(id=1)
c2 = Child(id=2)
c3 = Child(id=3)
 
p1.children = [c1, c2]
p2.children = [c2, c3]
 
session.add(p1)
session.add(p2)
session.commit()
 
print '{} childrens: {}'.format(p1, p1.children)
print '{} parents: {}'.format(c2, c2.parent)

<Parent(id=1)> childrens: [<Child(id=1)>, <Child(id=2)>]
<Child(id=2)> parents: [<Parent(id=1)>, <Parent(id=2)>]

c2 = session.query(Child).get(2)
print '{} parents: {}'.format(c2, c2.parent)

<Child(id=2)> parents: [<Parent(id=1)>, <Parent(id=2)>]