如何扩展 SQLAlchemy 的 association_proxy 的 getter 功能？

Question

编辑：我想 model User和Comment之间的1 to 0:1关系（用户可以有零个或一个评论）。 我不想访问 object Comment ，而是直接访问评论本身。 使用 SQLAlchemys association_proxy非常适合这种情况，除了一件事：在关联Comment之前访问User.comment 。 但在这种情况下，我宁愿期望None而不是AttributeError作为结果。

看下面的例子：

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，以下代码抛出一个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

您实际上不需要为此的association_proxy 。 拥有普通property你真的可以过得很好。 AttributeError是（可能）引起的，因为comment_object本身是None ，因为没有相关行，并且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

尝试这个

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

你给uselist在backref必须处于relationship 。

Answer 3

我没有看到任何可以解决问题的答案，例如也可以使用“sort_by”。

也许使用“column_property”会更好，请参阅按关联代理排序：无效 sql 。