On Feb 17, 2014, at 6:23 PM, Rob Crowell <robccrow...@gmail.com> wrote:
> I am having a bit of trouble getting DeferredReflection working the way I
> want; not sure if I am overlooking something obvious or if I just don't
> really understand how it's supposed to work.
>
> I'm trying to define my models before creating my engine (this does not work):
>
> Base = declarative_base(cls=DeferredReflection)
>
> class CityStats(Base):
> __tablename__ = 'city_stats'
> __table_args__ = {'schema': 'prod', 'autoload': True}
>
> if __name__ == '__main__':
> engine = create_engine('...')
> Base.metadata.bind = engine
> Base.prepare(engine)
>
>
> When I run this I get an error creating the CityStats class:
> "sqlalchemy.exc.UnboundExecutionError: No engine is bound to this Table's
> MetaData. Pass an engine to the Table via autoload_with=<someengine>, or
> associate the MetaData with an engine via metadata.bind=<someengine>”
its all about the stack trace, lets look:
Traceback (most recent call last):
File "test.py", line 8, in <module>
class CityStats(Base):
File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/ext/declarative/api.py",
line 53, in __init__
_as_declarative(cls, classname, cls.__dict__)
File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/ext/declarative/base.py",
line 251, in _as_declarative
**table_kw)
File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/sql/schema.py", line 350,
in __new__
table._init(name, metadata, *args, **kw)
File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/sql/schema.py", line 423,
in _init
self._autoload(metadata, autoload_with, include_columns)
File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/sql/schema.py", line 439,
in _autoload
msg="No engine is bound to this Table's MetaData. "
File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/sql/base.py", line 459, in
_bind_or_error
what we see here is that this script doesn’t get to create_engine() at all,
it’s trying to hit the database as soon as you say “CityStats(Base)”. Why is
that? Because you have “autoload=True” in your table args, which means,
“reflect this table *right now*”. That is, you are defeating the purpose of
using DeferredReflection.
The solution is just take out that autoload=True. Any class which descends
from the Base here is automatically part of the “classes to reflect” since you
have DeferredReflection at the base.
Also, if you’re on 0.9 take a look at the new “automap” extension, I’ve been
using it and it’s pretty keen. There’s one fix for it in not-yet-released
0.9.3 but it’s only needed if you’re dealing with inheritance structures.
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