Thanks Simon !
I got it working using the hack.
I'm not using the "mapper_configured" event because I need to add a
"parent" attribute to the subclass of TreeNode,
which should be done before the mapping.
On Friday, February 17, 2017 at 6:21:57 PM UTC+8, Simon King wrote:
>
> On Fri, Feb 17, 2017 at 10:03 AM, <linl...@gmail.com <javascript:>>
> wrote:
> > I want to achieve something like this:
> >
> > Base = declarative_base()
> >
> > # the abstract base, no table generated for it
> > class TreeNode(Base):
> > __abstract__ = True
> > parent = Column(ForeignKey('child_class_table.id'), nullable=False)
> >
> > def is_root(self):
> > pass
> > def get_parent(self):
> > pass
> >
> > # more methods
> >
> >
> > # the child, mapped to a table(in this case named 'discipline')
> > class Discipline(TreeNode):
> > __tablename__ = 'discipline'
> >
> > id = Column(Integer, primary_key=True)
> > name = Column(Unicode(500), nullable=False)
> >
> >
> > # a generated auxiliary table for the child(in this case named
> > 'discipline_closure')
> > class DisciplineClosure(Base):
> > __tablename__ = 'discipline_closure'
> >
> > ancestor_id = Column(ForeignKey('discipline.id'), nullable=False)
> > descendant_id = Column(ForeignKey('discipline.id'), nullable=False)
> > depth = Column(Integer, nullable=False)
> >
> > Is this possible in SQLAlchemy? How to implement it ?
> > Or is there an another way to implement my closure table ?
> >
> > My Django implementation Attached, but I wanna do it using SQLAlchemy.
> >
>
> You could look at the versioned_history example, which creates history
> tables for classes inheriting from a "Versioned" mixin:
>
>
> http://docs.sqlalchemy.org/en/latest/orm/examples.html#module-examples.versioned_history
>
>
> Here's the Versioned class:
>
> class Versioned(object):
> @declared_attr
> def __mapper_cls__(cls):
> def map(cls, *arg, **kw):
> mp = mapper(cls, *arg, **kw)
> _history_mapper(mp)
> return mp
> return map
>
> Declarative accesses the __mapper_cls__ attribute when creating the
> SQLAlchemy mapper for the class. The Versioned class uses that as a
> hook to generate the second class.
>
> This is a bit of a hack (particularly since __mapper_cls__ seems to be
> barely documented). It might be possible to do something similar using
> ORM events, but I haven't tried it. You could investigate the
> mapper_configured event:
>
>
> http://docs.sqlalchemy.org/en/latest/orm/events.html#sqlalchemy.orm.events.MapperEvents.mapper_configured
>
>
> The code would look something like:
>
> @event.listens_for(TreeNode, 'mapper_configured', propagate=True)
> def receive_mapper_configured(mapper, class_):
> # Create your Closure class here
>
> Hope that helps,
>
> Simon
>
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