On Jun 11, 2010, at 5:16 AM, jpeck wrote:
> I am trying to map to a simple read only property. According to the
> docs, I *think* I am supposed to use synonym. The problem is that I am
> getting a None value for the mapped descriptor's column.
>
> For example:
>
> import datetime
> from sqlalchemy import Column, Table, Integer, String, MetaData,
> create_engine
> from sqlalchemy.orm import mapper, sessionmaker, synonym
>
> meta = MetaData()
> foo_table = Table('foo', meta,
> Column('id', String(3), primary_key=True),
> Column('description', String(64), nullable=False),
> Column('calculated_value', Integer, nullable=False),
> )
>
> class Foo(object):
> def __init__(self, id, description):
> self.id = id
> self.description = description
>
> @property
> def calculated_value(self):
> self._calculated_value = datetime.datetime.now().second + 10
> return self._calculated_value
>
> mapper(Foo, foo_table,
> properties = {
> 'calculated_value' : synonym('_calculated_value',
> map_column=True)
> }
> )
wouldn't this be simpler ?
class Foo(object):
def __init__(self, id, description):
self.id = id
self.description = description
self.calculated_value = datetime.now().second + 10
mapper(Foo, foo_table)
want it read only ?
class Foo(object):
def __init__(self, id, description):
self.id = id
self.description = description
self._calculated_value = datetime.now().second + 10
@property
def calculated_value(self):
return self._calculated_value
mapper(Foo, foo_table,
properties={'_calculated_value':foo_table.c.calculated_value})
another option is to stick "datetime.now().second + 10" into theTable directly
as a "default" for the Column. If it were still empty at flush time, it would
get persisted at that point.
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