On Jun 11, 10:13 am, Michael Bayer <mike...@zzzcomputing.com> wrote:
> 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.
Michael - thanks for the response. My original post was just a
contrived example, and I should probably have included something
closer to what I was actually doing. In my production code I was using
the read only descriptor to iterate over a list of objects to
calculate a value. I need the property to calculate the value on the
fly in case something gets added/deleted from the list of objects. The
issue I had was that I needed the property to fire just before
insertion into the database to update the value referenced by synonym.
I ended up using MapperExtension to access the property before the
insertion occurred...
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