SQLAlchemy configuring many-to-many relationship to self using Association

Question

I am having problems configuring a many to many relationship to a model itself. I can configure a self-many-to-many relation when I use anormal relationship configuration i.e. one not using an Association object.

In this scenario I have to record some extra information in the many-to-many table itself so I'm trying to implement relationship using an Association object (PageLink).

Here are the models.

class PageLink(Base):
    '''
    Association table.
    '''
    __tablename__ = 'page_links'

    id = Column(Integer,primary_key=True)
    page_from = Column(Integer,ForeignKey('page.id'),primary_key=True)
    page_to = Column(Integer,ForeignKey('page.id'),primary_key=True)
    extra_col1 = Column(String(256),nullable=False)

class Page(Base):
    '''
    main table
    '''

    __tablename__ = 'page'

    id = Column(Integer,primary_key=True)
    name = Column(String(56),nullable=False)

    linked = relationship('PageLinks',backref='parent_page',
                          primaryjoin=id==PageLink.page_from,
                          secondaryjoin=id==PageLink.page_to)

This approach does not work. I have tried removing 'secondaryjoin' keyword but it wouldn't work.

Would greatly appreciate any help or advice on this matter.

Thank you for reading.

Answer 1

The association object pattern is not a sepecialization of the many-to-many relationship, but rather a special case of one-to-many relationships where you have a left_table - Many-To-One - association_table - One-To-Many - right_table set up. In short, you need two relationships, neither of which should have a secondary/secondaryjoin.

class PageLink(Base):
    '''
    Association table.
    '''
    __tablename__ = 'page_links'

    id = Column(Integer,primary_key=True)
    page_from = Column(Integer,ForeignKey('page.id'),primary_key=True)
    page_to = Column(Integer,ForeignKey('page.id'),primary_key=True)
    extra_col1 = Column(String(256),nullable=False)

class Page(Base):
    '''
    main table
    '''

    __tablename__ = 'page'

    id = Column(Integer,primary_key=True)
    name = Column(String(56),nullable=False)

    linked_to = relationship('PageLinks',backref='parent_page',
                             primaryjoin=id==PageLink.page_from)
    linked_from = relationship('PageLinks',backref='child_page',
                               primaryjoin=id==PageLink.page_to)

which means, to access the extra column for the 'to' links from some page p, you have to do: p.linked_to[0].extra_col1, or to get the actual linked page, p.linked_to[0].page_to

---

As an aside, it's often a great idea to use either an autoincrement primary key or (left/right) foreign key pair as the primary key in associations, but almost never useful to have both in the primary key. An alternative that combines both ideas would be to use an autoincrement integer as the only column in the primary key, and have an additional unique constraint on the left/right foreign key columns.