(moving to google groups)
the idea of using an instrumented list subclass is that you *would*
use the association object, and just have the list subclass handle
the references to the association object for you, i.e.:
class MyList(list):
def append(self, item):
super(MyList, self).append(new ItemAssociation(item))
def __getitem__(self, index):
return super(MyList, self).__getitem__(index).item
..etc
On Nov 1, 2006, at 3:15 PM, Karl Guertin wrote:
> I'm working on a database where the relation between users and objects
> is defined by an association table that looks like:
>
> role_objects_table = Table("role_objects", metadata,
> Column("objects_id", Integer,
> ForeignKey("objects.objects_id"),
> primary_key=True),
> Column("role",Role_Type),
> Column("user_id", Integer,
> ForeignKey("tg_user.user_id"),
> primary_key=True))
>
> Where Role_Type is an Enum class that translates roles onto Unicode
> (1).
>
> And Objects is mapped like so:
>
> assign_mapper(session.context, Objects, objects_table, properties =
> dict(
> #...
> responsibles = relation(User, secondary=role_objects_table,
> primaryjoin = and_(objects_table.c.objects_id ==
> role_objects_table.c.objects_id,
> role_objects_table.c.role == 'R')),
> approvers = relation(User, secondary=role_objects_table,
> primaryjoin = and_(objects_table.c.objects_id ==
> role_objects_table.c.objects_id,
> role_objects_table.c.role == 'A')),
> #...
> )
>
> So basically the relationship between users and objects is typed by
> the role field. This is a textbook use case for the use of an
> association object, but using an association object is really
> annoying. I would really like to be able to treat each relation as a
> normal many to many relation and not have to create intermediate
> objects just to set one field to a constant value.
>
> In looking through the archives I found:
>
>> You could go more nuts with [object properties] by having the list
>> be a
>> subclass of "list" which properly does mutation operations too,
>> creates
>> the association objects automatically, etc.
>
> Which seems to be what I'm looking to do. I'm assuming that the "list"
> referenced here is custom collection_class, but I don't see any
> obvious hooks into the secondary table and trying to manage the state
> of intermediate objects is more work than I'd like.
>
> In looking through the code, I noticed
> sqlalchemy.orm.attributes.AttributeExtension, which makes it pretty
> easy to hook into the event I'm interested in, but I'm having trouble
> finding a reference to the secondary table.
>
> Any guidance would be appreciated.
>
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> ---
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