Re: [sqlalchemy] How to map to read only descriptor
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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Re: [sqlalchemy] How to map to read only descriptor
On Fri, Jun 11, 2010 at 4:16 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. > Ok, so I got this working with synonym, but I am not sure I am doing this the best way. As long as I access the descriptor before an insert or update, the synonym worked ok. I ended up using a MapperExtension like so: class UpdatePropertiesExtension(MapperExtension): def __init__(self, properties): self.properties = properties def _update_properties(self, instance): for p in self.properties: getattr(instance, p) def before_insert(self, mapper, connection, instance): self._update_properties(instance) def before_update(self, mapper, connection, instance): self._update_properties(instance) This works fine for me, but I'm wondering if there is a better way to do this? -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
[sqlalchemy] How to map to read only descriptor
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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