“指向”
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即定义一张表中的数据条目指向另一张表中的条目，建立这种有向的“指向”可以让表以字段的方式查询到被指向的条目(们)，所以，如果要双向查询，就需要双向指向。
One To Many
在“多”方表中添加“一”方的id作为ForeignKey约束，为查询方便双方均需要定义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
与One To Many区别不大，只要“一”方的relationship方法中添加一个"uselist=False"参数即可，uselist是一个标量属性(a scalar attribute)，其含义是“一”方对应另一张表的条目不使用列表。
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')) # 多方才有这个字段
parent = relationship("parent", back_populates="child")
# 通常也会把backref用函数表示天机uselist=False参数，用以显示指出。
# parent = rlationship("parent", backref=backref("child", uselist=False))Many To Many
借助中间表完成，利用relationship支持的secondary参数，
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."即接受可执行参数，可以让association_table 在稍晚的时候定义，甚至可以在所有模块都初始化完成后，直到它可调用为止。
class parent(Base):
__tablename__ = "left"
id = Column(Integer, primary_key=True)
children = relationship("Child",
secondary=lambda: assciation_table,
backref="parents")以上relationship参数的表明均可以用类名字符串代替。
删除Mang To Many记录，不必手动删除secondary的中间表数据，数据库会根据“cascade rule”级联规则自动删除。
如果中间表对象需要被调用
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)# 寻找答案的路途上要保持耐心和专心！
需要注意：back_populates参数赋值参数一定不能是relationship第一个参数的字段，那样相当于被对应关系表中有了重复字段。
