On May 19, 2011, at 5:04 AM, Faheem Mitha wrote:
>
> from sqlalchemy import *
> from sqlalchemy.orm import *
>
> def make_pheno_table(meta, schema, name='pheno'):
> pheno_table = Table(
> name, meta,
> Column('patientid', String(60), primary_key=True),
> schema=schema,
> )
> return pheno_table
>
> class Pheno(object):
> def __init__(self, patientid):
> self.patientid = patientid
>
> def make_tables(schema):
> from sqlalchemy import MetaData
> meta = MetaData()
> pheno_table = make_pheno_table(meta, schema)
> mapper(Pheno, pheno_table)
> table_dict = {'metadata': meta, 'pheno_table':pheno_table}
> return table_dict
>
> table_dict = make_tables('foo')
> table_dict = make_tables('bar')
I'm sure I mentioned earlier, one of the reasons you're finding this difficult
is because of this type of pattern, where you're calling mapper() and Table at
a different scope than where you make your classes i.e. within a callable
versus module level. At the very least a function like make_tables() would
ensure that its internal implementation is called only once, such as:
_table_dict = None
def make_tables():
global _table_dict
if _table_dict is not None:
return _table_dict
_table_dict = {.... }
otherwise, you're creating new MetaData objects each time, rebuilding a whole
new set of Tables in it each time, i.e. its a total mess. Engine, Session
registry, MetaData, your set of classes and their mappers, these should all be
global objects, created once per application. If you want to use functions to
initiate their existence that is fine but you should be maintaining singleton
behavior in any case.
>
> from sqlalchemy.orm.exc import UnmappedClassError
>
> try:
> class_mapper(Pheno)
> except UnmappedClassError:
> mapper(Pheno, pheno_table)
>
> *********************************************************************
>
> If a mapper is not defined for Pheno, it throws an
> UnmappedClassError. This at least doesn't return an error in my test
> script, but I haven't checked if it actually works.
It will work, but a more reasonable design wouldn't need such a pattern.
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