从超类实例化一个子类

Question

I have the following python code that creates a polymorphic structure in a MySQL DB using SQLAlchemy.

class Animal(Base):
    key = Column(Integer(), Sequence("My Counter" ,1 ,1), primary_key = True)
    name = Column(String())
    discriminator = Column('type',String())
    __mapper_args__ = {'polymorphic_on':discriminator}

    def __init__(self,key,name):
        self.key = key
        self.name = name

class Cat(Animal):
    __tablename__ = 'cat'
    __mapper_args__ = {'polymorphic_identity':'cat'}
    def walk():
        pass

class Dog(Animal):
    __tablename__ = 'dog'
    __mapper_args__ = {'polymorphic_identity':'dog'}

    def walk():
         pass

I was wondering what the best way is to load it such that:

a = Animal(key=1)
c = a.create()

c would now be a Cat or a Dog object depending on what type it is. The Animal table has this information.

Thanks

Answer 1

This is not how you query the database using sqlalchemy. In order to get an Animal for specific key all you need to do is to perform:

my_key = 1
a = session.query(Animal).get(my_key)
# sqlalchemy will figure out the type automatically and will return object of proper class 
assert type(a) in (Cat, Dog,)

Please note that the model your provided is incomplete. The one below should be a complete working one:

class Animal(Base):
    __tablename__ = 'animal'
    key = Column(Integer(), Sequence("My Counter" ,1 ,1), primary_key = True)
    name = Column(String())
    discriminator = Column('type',String())
    __mapper_args__ = { 'polymorphic_on':discriminator, }

    def __init__(self,key,name):
        self.key = key
        self.name = name

class Cat(Animal):
    __tablename__ = 'cat'
    __mapper_args__ = {'polymorphic_identity':'cat'}
    key = Column(Integer(), ForeignKey('animal.key'), primary_key = True)

    def walk(self):
        print "cat walking"

class Dog(Animal):
    __tablename__ = 'dog'
    __mapper_args__ = {'polymorphic_identity':'dog'}
    key = Column(Integer(), ForeignKey('animal.key'), primary_key = True)

    def walk(self):
        print "dog walking"

Answer 2

I don't really know exactly how to apply this to what you wrote, but I have done something similar.

You will want to create a factory function; this will probably be in your base class, though it could probably be separate. in any case, it needs to be a static function, so if you include it in your class, declare it @staticmethod .

Second, you can access the __subclasses__ method eg BaseClass.__subclasses__() to get a list of actual subclasses of BaseClass . In my case, I scanned the subclasses looking for one that had a specific class attribute and then called its init function.