如何扩展SQLAlchemy 的association_proxy 的`getter` 功能? [英] How to extend the `getter`-functionality of SQLAlchemy's association_proxy?

问题描述

我想对 User 和 Comment 之间的 1 to 0:1 关系建模(一个用户可以有零个或一个评论).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.

I would like to model a 1 to 0:1 relationship between User and Comment (a User can have zero or one Comment). Instead of accessing the object Comment I would rather directly access the comment itself. Using SQLAlchemys association_proxy works perfect for that scenario except for one thing: accessing User.comment before having a Comment associated. But in this case I would rather expect None instead of AttributeError as result.

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，下面的代码抛出一个AttributeError:

Now, the following code throws an 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?

What would be the best way to catch that exception and provide a default value instead (like an empty string)?

推荐答案

你真的需要 association_proxy 来解决这个问题.使用常规的属性，您真的可以过得很好.AttributeError 是(可能)引起的，因为 comment_object 本身是 None，因为没有依赖行，并且 None 没有 comment 属性.

You don't really need association_proxy for this. You could really get by just fine with a regular property. The AttributeError is (probably) caused because the comment_object is itself None, since there is no dependent row, and None has no comment attribute.

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

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