I am new to programming and am following the example in the Pylons documentation on creating a Wiki. The database I want to link to the wiki was created with Elixir so I rewrote the Wiki database schema and have continued from there. In the wiki there is a requirement for a Navigation table which is inherited by Pages and Sections. A section can have many pages, while a page can only have one section. In addition, each sibling node can be chain-referenced to each other. So: Nav has "section" (OneToMany) and "before" (OneToOne - to reference preceeding node) Page has "section" (ManyToOne - many pages in one section) and inherits "before" Section inherits all from Nav The code I've written looks like this: class Nav(Entity): using_options(inheritance='multi') name = Field(Unicode(30), default=u'Untitled Node') path = Field(Unicode(255), default=u'') section = OneToMany('Page', inverse='section') after = OneToOne('Nav', inverse='before') before = OneToMany('Nav', inverse='after') class Page(Nav): using_options(inheritance='multi') content = Field(UnicodeText, nullable=False) posted = Field(DateTime, default=now()) title = Field(Unicode(255), default=u'Untitled Page') heading = Field(Unicode(255)) tags = ManyToMany('Tag') comments = OneToMany('Comment') section = ManyToOne('Nav', inverse='section') class Section(Nav): using_options(inheritance='multi') Errors received on this: sqlalchemy.exc.OperationalError: (OperationalError) table nav has no column named aftr_id u'INSERT INTO nav (name, path, aftr_id, row_type) VALUES (?, ?, ?, ?)' I've also tried: before = ManyToMany('Nav', inverse='before') on Nav in the hopes this might break the problem, but also not. The original SQLAlchemy code from the tutorial for these declarations is as follows: nav_table = schema.Table('nav', meta.metadata, schema.Column('id', types.Integer(), schema.Sequence('nav_id_seq', optional=True), primary_key=True), schema.Column('name', types.Unicode(255), default=u'Untitled Node'), schema.Column('path', types.Unicode(255), default=u''), schema.Column('section', types.Integer(), schema.ForeignKey('nav.id')), schema.Column('before', types.Integer(), default=None), schema.Column('type', types.String(30), nullable=False) ) page_table = schema.Table('page', meta.metadata, schema.Column('id', types.Integer, schema.ForeignKey('nav.id'), primary_key=True), schema.Column('content', types.Text(), nullable=False), schema.Column('posted', types.DateTime(), default=now), schema.Column('title', types.Unicode(255), default=u'Untitled Page'), schema.Column('heading', types.Unicode(255)), ) section_table = sa.Table('section', meta.metadata, schema.Column('id', types.Integer, schema.ForeignKey('nav.id'), primary_key=True), ) orm.mapper(Nav, nav_table, polymorphic_on=nav_table.c.type, polymorphic_identity='nav') orm.mapper(Section, section_table, inherits=Nav, polymorphic_identity='section') orm.mapper(Page, page_table, inherits=Nav, polymorphic_identity='page', properties={ 'comments':orm.relation(Comment, backref='page', cascade='all'), 'tags':orm.relation(Tag, secondary=pagetag_table) }) Any help is much appreciated.