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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