SQLAlchemy - how to map against a read-only (or calculated) property
- by Jeff Peck
I'm trying to figure out how to map against a simple read-only property and have that property fire when I save to the database.
A contrived example should make this more clear. First, a simple table:
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),
)
What I want to do is set up a class with a read-only property that will insert into the calculated_value column for me when I call session.commit()...
import datetime
def 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
According to the sqlalchemy docs, I think I am supposed to map this like so:
mapper(Foo, foo_table, properties = {
'calculated_value' : synonym('_calculated_value', map_column=True)
})
The problem with this is that _calculated_value is None until you access the calculated_value property. It appears that SQLAlchemy is not calling the property on insertion into the database, so I'm getting a None value instead. What is the correct way to map this so that the result of the "calculated_value" property is inserted into the foo table's "calculated_value" column?
