是否可以在SQLAlchemy中创建Column,它将在上次插入/更新时自动填充时间？

Question

是否可以在SQLAlchemy中创建Column,它将在上次插入/更新时自动填充时间？我创建了继承自Base类的模型

class Base(object):
    def __tablename__(self):
        return self.__name__.lower()
    id = Column(Integer, primary_key=True)
    last_time = Column(TIMESTAMP, server_default=func.now())

Base = declarative_base(cls=Base)

class EntityModel(Base):
    __tablename__ = 'entities'
    settlement_id = Column(Integer, ForeignKey('settlements.id'), nullable=False)
    type = Column(String(20), nullable=False)
    level = Column(Integer, nullable=False, default=0)
    energy = Column(Float, nullable=False, default=0)
    position_x = Column(Integer, default=0)
    position_y = Column(Integer, default=0)

    def __repr__(self):
        return "<Entity('%s')>" % (self.type)

每当我更新EntityModel时,我想在系统function.now()上使用last_time.我可以使用触发器在数据库级别上执行此操作,但我宁愿在应用程序级别上执行此操作,如果可能的话.

Answer 1

在Base类中,在最后一个语句中添加onupdate,如下所示:

last_time = Column(TIMESTAMP, server_default=func.now(), onupdate=func.current_timestamp())

Answer 2

如果您使用MySQL,我相信您只能有一个自动更新日期时间列,因此我们使用SQLAlchemy的事件触发器.

您只需将一个侦听器附加到'before_insert'和'before_update'挂钩并根据需要进行更新:

from sqlalchemy import event

@event.listen(YourModel, 'before_insert')
def update_created_modified_on_create_listener(mapper, connection, target):
  """ Event listener that runs before a record is updated, and sets the create/modified field accordingly."""
  target.created = datetime.utcnow()
  target.modified = datetime.utcnow()

@event.listen(YourModel, 'before_update')
def update_modified_on_update_listener(mapper, connection, target):
  """ Event listener that runs before a record is updated, and sets the modified field accordingly."""
  # it's okay if this field doesn't exist - SQLAlchemy will silently ignore it.
  target.modified = datetime.utcnow()

我知道没有人会记得将它添加到新模型中,所以我试着变得聪明并为它们添加它.

我们所有的模型都继承自我们巧妙地称为"DatabaseModel"的基础对象.我们检查谁从这个对象继承并动态地将触发器添加到所有这些对象.

如果模型没有创建或修改的字段,那就没关系 - SQLAlchemy似乎默默地忽略它.

class DatabaseModel(db.Model):
  __abstract__ = True

  #...other stuff...

  @classmethod
  def _all_subclasses(cls):
    """ Get all subclasses of cls, descending. So, if A is a subclass of B is a subclass of cls, this
    will include A and B.
    (Does not include cls) """
    children = cls.__subclasses__()
    result = []
    while children:
      next = children.pop()
      subclasses = next.__subclasses__()
      result.append(next)
      for subclass in subclasses:
        children.append(subclass)
    return result

def update_created_modified_on_create_listener(mapper, connection, target):
  """ Event listener that runs before a record is updated, and sets the create/modified field accordingly."""
  # it's okay if one of these fields doesn't exist - SQLAlchemy will silently ignore it.
  target.created = datetime.utcnow()
  target.modified = datetime.utcnow()

def update_modified_on_update_listener(mapper, connection, target):
  """ Event listener that runs before a record is updated, and sets the modified field accordingly."""
  # it's okay if this field doesn't exist - SQLAlchemy will silently ignore it.
  target.modified = datetime.utcnow()


for cls in DatabaseModel._all_subclasses():
  event.listen(cls, 'before_insert',  update_created_modified_on_create_listener)
  event.listen(cls, 'before_update',  update_modified_on_update_listener)