如何扩展 SQLAlchemy 的关联代理的 `getter` 功能？

Question

编辑：我想在用户和评论之间建立一个1到0:1关系（用户可以有零或一个评论）。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

为此，您实际上并不需要association_proxy。您确实可以通过常规属性过得很好。 AttributeError 的原因（可能）是因为 comment_object 本身是 None，因为没有依赖行，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

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 backref 中给出了 uselist，它必须处于 relationship 中。

Answer 3

我没有看到任何可以解决问题并且也可以与“sort_by”一起使用的答案。

也许使用“column_property”更好，请参阅按关联代理排序：无效的sql< /a>.