Awesome, both look good, will try them out. Thanks for the quick reply.
On Thursday, March 17, 2016 at 3:54:11 PM UTC-6, Mike Bayer wrote:
>
> Just core, you can some_table.append_column(Column(... whatever...)) at
> any time. The only "event" I can think of are the DDL attachment
> events:
>
> from sqlalchemy import *
> from sqlalchemy import event
>
> m = MetaData()
>
>
> table_a = Table(
> 'a', m,
> Column('id', Integer)
> )
>
>
> @event.listens_for(Table, "after_parent_attach")
> def evt(target, parent):
> if parent is m and target.name == 'b':
> m.tables['a'].append_column(
> Column('data', Integer, default=select([target]).as_scalar())
> )
>
> table_b = Table(
> 'b', m,
> Column('id', Integer)
> )
>
> e = create_engine("sqlite://", echo=True)
> m.create_all(e)
>
> e.execute(table_a.insert())
>
>
> I guess if you really want to get the column default to be something
> that becomes the SQL at the last minute, you could do something like this:
>
> from sqlalchemy import *
> from sqlalchemy.sql import ColumnElement
> from sqlalchemy.ext.compiler import compiles
>
>
> class MyThing(ColumnElement):
> pass
>
>
> @compiles(MyThing)
> def _generate_my_thing(element, compiler, **kw):
> return compiler.process(select([table_b]).as_scalar())
>
> m = MetaData()
>
>
> table_a = Table(
> 'a', m,
> Column('id', Integer),
> Column('data', Integer, default=MyThing())
> )
>
> table_b = Table(
> 'b', m,
> Column('id', Integer)
> )
>
> e = create_engine("sqlite://", echo=True)
> m.create_all(e)
>
> e.execute(table_a.insert())
>
>
>
>
>
> On 03/17/2016 05:25 PM, Jonathan Beluch wrote:
> > We're just using core, is there some equivalent?
> >
> > On Thursday, March 17, 2016 at 3:19:23 PM UTC-6, Mike Bayer wrote:
> >
> >
> >
> > On 03/17/2016 04:47 PM, Jonathan Beluch wrote:
> > > Background: Using core we have tables defined in a few separate
> > files.
> > > Goal: To have column defaults be selectables which reference
> other
> > > tables while avoiding circular imports. To avoid circular imports
> I
> > > cannot always build the selects at import time, they have to be
> > > generated inside a function that takes table/col names similar to
> > how
> > > FKs work.
> > >
> > > It seems that a callable for Column.default cannot return an
> > uncompiled
> > > statement. Two solutions I see:
> > >
> > > 1) Use the context provided to default callables to compile the
> > dynamic
> > > select statement.
> > > 2) Implement something similar to FKs, using all the parent
> attach
> > > events to set .arg to a selectable on a subclass of
> ColumnDefault.
> > >
> > > Thoughts?
> >
> > A column default callable is expected to produce the value that's to
> be
> > embedded into the INSERT values. You can execute any SQL you'd
> like
> > there, but that's after the INSERT statement's string form is
> already
> > decided.
> >
> > If the goal is that the default is a SQL clause to be embedded in
> the
> > string form of the INSERT, then you use a fixed default that is
> > represented by that SQL clause.
> >
> > If you can't generate that SQL clause due to imports, there's
> various
> > ways to defer the production of the Column but the simplest way, not
> > necessarily the nicest, is to stick it in declare_first:
> >
> > from sqlalchemy import *
> > from sqlalchemy.orm import *
> > from sqlalchemy.ext.declarative import declarative_base
> >
> > Base = declarative_base()
> >
> >
> > class B(Base):
> > __tablename__ = 'b'
> > id = Column(Integer, primary_key=True)
> >
> > @classmethod
> > def __declare_first__(cls):
> > cls.b = Column(Integer, default=select([A]).as_scalar())
> >
> >
> > class A(Base):
> > __tablename__ = 'a'
> > id = Column(Integer, primary_key=True)
> >
> > e = create_engine("sqlite://", echo=True)
> > configure_mappers()
> > Base.metadata.create_all(e)
> >
> > s = Session(e)
> > s.add(B())
> > s.commit()
> >
> >
> > __declare_first__ is actually just a hook for the before_configured
> > event so that's actually using the events in any case.
> >
> >
> >
> >
> > >
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