如何避免在SQLAlchemy - python中的多对多关系表中添加重复项？

Question

我正在处理与sqlalchemy的多对多关系.我的问题是如何避免在多对多关系表中添加重复的对值.

为了使事情更清楚,我将使用官方SQLAlchemy文档中的示例.

Base = declarative_base()

Parents2children = Table('parents2children', Base.metadata,                                                                                                                                                                                                     
  Column('parents_id', Integer, ForeignKey('parents.id')),                                                                                                                                                                                                       
  Column('children_id', Integer, ForeignKey('children.id'))
)

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)
    parent_name = Column(String(45))
    child_rel = relationship("Child", secondary=Parents2children, backref= "parents_backref") 

    def __init__(self, parent_name=""):
        self.parent_name=parent_name
    def __repr__(self):
        return "<parents(id:'%i', parent_name:'%s')>" % (self.id, self.parent_name)

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    child_name = Column(String(45))

    def __init__(self, child_name=""):
        self.child_name= child_name
    def __repr__(self):
        return "<experiments(id:'%i', child_name:'%s')>" % (self.id, self.child_name)

###########################################

def setUp():
    global Session
    engine=create_engine('mysql://root:root@localhost/db_name?charset=utf8', pool_recycle=3600,echo=False)
    Session=sessionmaker(bind=engine)

def add_data():
    session=Session()
    name_father1=Parent(parent_name="Richard")
    name_mother1=Parent(parent_name="Kate")
    name_daughter1=Child(child_name="Helen")
    name_son1=Child(child_name="John")

    session.add(name_father1)
    session.add(name_mother1)

    name_father1.child_rel.append(name_son1)
    name_daughter1.parents_backref.append(name_father1)
    name_son1.parents_backref.append(name_father1)

    session.commit()
    session.close()


setUp()
add_data()
session.close()

使用此代码,表中插入的数据如下:

父母表:

+----+-------------+
| id | parent_name |
+----+-------------+
|  1 | Richard     |
|  2 | Kate        |
+----+-------------+

儿童桌:

+----+------------+
| id | child_name |
+----+------------+
|  1 | Helen      |
|  2 | John       |
+----+------------+

Parents2children表

+------------+-------------+
| parents_id | children_id |
+------------+-------------+
|          1 |           1 |
|          1 |           2 |
|          1 |           1 |
+------------+-------------+

如您所见,最后一个表中有一个副本...我怎么能阻止SQLAlchemy添加这些重复项？

我试图把关系("Child",secondary = ...,collection_class = set)但是会显示以下错误:

AttributeError: 'InstrumentedSet' object has no attribute 'append'

Answer 1

将PrimaryKeyConstraint(或UniqueConstraint)添加到relationship表中:

Parents2children = Table('parents2children', Base.metadata,                                                                                                                                                                                                     
  Column('parents_id', Integer, ForeignKey('parents.id')),                                                                                                                                                                                                       
  Column('children_id', Integer, ForeignKey('children.id')),
  PrimaryKeyConstraint('parents_id', 'children_id'),
)

当您尝试提交从双方添加的关系时,您的代码将生成错误.这是非常值得推荐的.

为了不产生错误,请先检查:

if not(name_father1 in name_son1.parents_backref):
    name_son1.parents_backref.append(name_father1)