“指向”
即定義一張表中的數據條目指向另一張表中的條目，建立這種有向的“指向”可以讓表以字段的方式查詢到被指向的條目(們)，所以，如果要雙向查詢，就需要雙向指向。
One To Many
在“多”方表中添加“一”方的id作爲ForeignKey約束，爲查詢方便雙方均需要定義relationship()字段；
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
children = relationship("Child", back_populates="parent")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id'))
parent = relationship("Parent", back_populates="children")One To One
與One To Many區別不大，只要“一”方的relationship方法中添加一個"uselist=False"參數即可，uselist是一個標量屬性(a scalar attribute)，其含義是“一”方對應另一張表的條目不使用列表。
class Parent(Base):
__tablename__ = 'parent'
id = Column(Integer, primary_key=True)
child = relationship("Child", uselist=False, back_populates="parent")
class Child(Base):
__tablename__ = 'child'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id')) # 多方纔有這個字段
parent = relationship("parent", back_populates="child")
# 通常也會把backref用函數表示天機uselist=False參數，用以顯示指出。
# parent = rlationship("parent", backref=backref("child", uselist=False))Many To Many
藉助中間表完成，利用relationship支持的secondary參數，
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,
back_populates="parents")
class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
parents = relationship("Parent",
secondary=association_table,
back_populates="children")注意，secondary可以接受'a callable that returns the ultimate argument,which is evaluated only when mappers are first used."即接受可執行參數，可以讓association_table 在稍晚的時候定義，甚至可以在所有模塊都初始化完成後，直到它可調用爲止。
class parent(Base):
__tablename__ = "left"
id = Column(Integer, primary_key=True)
children = relationship("Child",
secondary=lambda: assciation_table,
backref="parents")以上relationship參數的表明均可以用類名字符串代替。
刪除Mang To Many記錄，不必手動刪除secondary的中間表數據，數據庫會根據“cascade rule”級聯規則自動刪除。
如果中間表對象需要被調用
class Association(Base):
__tablename__ = 'association'
left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
extra_data = Column(String(50))
child = relationship("Child", back_populates="parents")
parent = relationship("Parent", back_populates="children")
class Parent(Base):
__tablename__ = 'left'
id = Column(Integer, primary_key=True)
children = relationship("Association", back_populates="parent")
class Child(Base):
__tablename__ = 'right'
id = Column(Integer, primary_key=True)
parents = relationship("Association", back_populates="child")
# create parent, append a child via association
p = Parent()
a = Association(extra_data="some data")
a.child = Child()
p.children.append(a)
# iterate through child objects via association, including association
# attributes
for assoc in p.children:
print(assoc.extra_data)
print(assoc.child)# 尋找答案的路途上要保持耐心和專心！
需要注意：back_populates參數賦值參數一定不能是relationship第一個參數的字段，那樣相當於被對應關係表中有了重複字段。