On Aug 28, 2010, at 1:58 PM, Adrià Mercader wrote:
> Hi everybody,
> I'm using SQLAlchemy for the first time, and everything worked as a
> charm until I got stuck with a double join.
> That's the expression I'm trying to perform to get a list of the
> linked articles from a given article, (knowing its title):
>
> SELECT a2.id,a2.title
> FROM articles a1 JOIN links l
> ON a1.id = l.id_parent
> JOIN articles a2 ON a2.id = l.id_child
> WHERE a1.title = 'Alabama'
>
> I have defined the two classes declaratively as follows:
>
> class Article(Base):
> __tablename__ = 'articles'
> id = Column(Integer, primary_key=True)
> title = Column(Unicode, nullable=False)
>
> class Link(Base):
> __tablename__ = 'links'
> id_parent = Column(Integer, ForeignKey("articles.id"),
> primary_key=True)
> id_child = Column(Integer, ForeignKey("articles.id"),
> primary_key=True)
>
> But when I try to execute the following code:
>
> links = session.query(Article).join((Link,Article.id ==
> Link.id_parent)).join((Article,Article.id == Link.id_child)).all()
>
> fails with the following error:
> (ProgrammingError) table name "articles" specified more than once
>
> I don't know if the problem it's in the model definition or in the
> query code, so any help will be greatly appreciated.
you need to use aliased() - see the ORM tutorial section "Using Aliases" for an
example.
>
> Thanks,
>
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