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POHow to extend the `getter`-functionality of SQLAlchemy's association_proxy?
 

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  1. POHow to extend the `getter`-functionality of SQLAlchemy's association_proxy?
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    <p><strong>Edit:</strong> I would like to model a <code>1 to 0:1</code> relationship between <code>User</code> and <code>Comment</code> (a User can have zero or one Comment). Instead of accessing the object <code>Comment</code> I would rather directly access the comment itself. Using SQLAlchemys <code>association_proxy</code> works perfect for that scenario <strong>except</strong> for one thing: accessing <code>User.comment</code> before having a <code>Comment</code> associated. But in this case I would rather expect <code>None</code> instead of <code>AttributeError</code> as result.</p> <p>Look at the following example:</p> <pre><code>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() </code></pre> <p>Now, the following code throws an <code>AttributeError</code>:</p> <pre><code>u = User(name="Max Mueller") print u.comment </code></pre> <p>What would be the best way to catch that exception and provide a default value instead (like an empty string)?</p>
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    1. COMaybe one solution would be to provide a specific `getset_factory`-parameter calling the `association_proxy`. But how?
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    2. COIs this really a `1-to-1` relationship (User can have only one Comment, as implied by your model)?
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    3. COIt looks to me that you should move the `uselist=False` outside of `backref` definition so that it applies to the `user_object` side of the relationship. Then you should rename the `comment` to `comments` and it will be a collection instead of a scalar object, so there should not be any exception.
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