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)
}
)
engine = create_engine('sqlite:///test.db')
Session = sessionmaker(bind=engine)
meta.create_all(bind=engine)
session = Session()
session.add( Foo('xyz', 'test only') )
session.commit()
Since self._calculated_value is created within the property, it does
not exist until you actually access the property. How do I set this up
to get sqlalchemy to go use the actual return value of the property
when session.commit is called?
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