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?
OK - I am editing this post in case someone else has the same question. What I ended up doing was using a MapperExtension. Let me give you a better example along with usage of the extension:
class UpdatePropertiesExtension(MapperExtension):
def __init__(self, properties):
self.properties = properties
def _update_properties(self, instance):
# We simply need to access our read only property one time before it gets
# inserted into the database.
for property in self.properties:
getattr(instance, property)
def before_insert(self, mapper, connection, instance):
self._update_properties(instance)
def before_update(self, mapper, connection, instance):
self._update_properties(instance)
And this is how you use this. Lets say you have a class with several read only properties that must fire before insertion into the database. I am assuming here that for each one of these read only properties, you have a corresponding column in the database that you want populated with the value of the property. You are still going to set up a synonym for each property, but you use the mapper extension above when you map the object:
class Foo(object):
def __init__(self, id, description):
self.id = id
self.description = description
self.items = []
self.some_other_items = []
@property
def item_sum(self):
self._item_sum = 0
for item in self.items:
self._item_sum += item.some_value
return self._item_sum
@property
def some_other_property(self):
self._some_other_property = 0
.... code to generate _some_other_property on the fly....
return self._some_other_property
mapper(Foo, metadata,
extension = UpdatePropertiesExtension(['item_sum', 'some_other_property']),
properties = {
'item_sum' : synonym('_item_sum', map_column=True),
'some_other_property' : synonym('_some_other_property', map_column = True)
})