Question

修改：我想在1 to 0:1和User之间建立Comment关系模型（用户可以有一个或一个评论）。我宁愿直接访问评论本身，而不是访问对象Comment。使用SQLAlchemys association_proxy非常适合该场景除之外的一件事：在关联User.comment之前访问Comment。但在这种情况下，我宁愿期待None而不是AttributeError作为结果。

请看以下示例：

import sqlalchemy as sa
import sqlalchemy.orm as orm
from sqlalchemy import Column, Integer, Text, ForeignKey, Table
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy

Base = declarative_base()

class User(Base):
  __tablename__ = 'users'
  id = Column(Integer, primary_key=True)
  name = Column(Text)

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

  # proxy the 'comment' attribute from the 'comment_object' relationship
  comment = association_proxy('comment_object', 'comment')


class Comment(Base):
  __tablename__ = 'comments'
  id = Column(Integer, primary_key=True)
  comment = Column('comment', Text, nullable=False, default="")
  user_id = Column(ForeignKey('users.id'), nullable=False, unique=True)
  # user_id has to be unique to ensure that a User can not have more than one comments

  def __init__(self, comment):
      self.comment = comment

  user_object = orm.relationship(
      "User",
      uselist=False, # added after edditing the question
      backref=orm.backref('comment_object', uselist=False)
      )

if __name__ == "__main__":
  engine = sa.create_engine('sqlite:///:memory:', echo=True)
  Session = orm.sessionmaker(bind=engine)
  Base.metadata.create_all(engine)
  session = Session()

现在，以下代码抛出AttributeError：

u = User(name="Max Mueller")
print u.comment

捕获该异常并提供默认值（如空字符串）的最佳方法是什么？

Answer 1

你真的不需要 association_proxy。你可以通过常规的property得到很好的结果。 AttributeError是（可能）导致的，因为comment_object本身是None，因为没有相关行，None没有comment属性。

class User(Base): __tablename__ = 'users' id = Column(Integer, primary_key=True) name = Column(Text) def __init__(self, name): self.name = name # proxy the 'comment' attribute from the 'comment_object' relationship @property def comment(self): if self.comment_object is None: return "" else: return self.comment_object.comment @comment.setter def comment(self, value): if self.comment_object is None: self.comment_object = Comment() self.comment_object.comment = value

Answer 2

试试这个

import sqlalchemy as sa
import sqlalchemy.orm as orm
from sqlalchemy import Column, Integer, Text, ForeignKey, Table
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy

Base = declarative_base()

class User(Base):
  __tablename__ = 'users'
  id = Column(Integer, primary_key=True)
  name = Column(Text)

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

  # proxy the 'comment' attribute from the 'comment_object' relationship
  comment = association_proxy('comment_object', 'comment')


class Comment(Base):
  __tablename__ = 'comments'
  id = Column(Integer, primary_key=True)
  comment = Column('comment', Text, nullable=False, default="")
  user_id = Column(ForeignKey('users.id'), nullable=False)

  def __init__(self, comment):
      self.comment = comment

  user_object = orm.relationship(
      "User",
      backref=orm.backref('comment_object'),
      uselist=False
      )

if __name__ == "__main__":
  engine = sa.create_engine('sqlite:///:memory:', echo=True)
  Session = orm.sessionmaker(bind=engine)
  Base.metadata.create_all(engine)
  session = Session()
  u = User(name="Max Mueller")
# comment = Comment("")
# comment.user_object = u

# session.add(u)
# session.commit()
  print "SS :", u
  print u.comment

您在uselist中提供的backref必须位于relationship。

如何扩展SQLAlchemy的association_proxy的`getter`功能？

2 个答案: