David Cramer wrote:
> I would say ignore triggers on the DB level, until they've been
> written in the framework.
>
Yes, this was essentially my point earlier; triggers would be nice to
have from a consistency point of view, but it will be easier and quicker
to reimplement them in Python, not least because it allows more complex
functionality to be put straight in if we want it in the future. Then,
we can always extend it so that some types of fields are implemented
directly as triggers if you want once the API is settled.
With all this in mind, my API proposal would be something like this:
A CopiedField, called like so: user_username =
models.CopiedField("user", "username") [here, "user" is the name of
the ForeignKey to link through]
An AggregateField: post_count =
models.AggregateField(Post.objects.filter(visible=True), "count")
The CopiedField essentially works how my current DenormField does, but
using a foreignkey to avoid you having to specify both it _and_ the
model if you have more than one to the same model (a reasonable amount
of the DenormField code is just for finding which foreignkey to link
through, anyway)
The AggregateField takes a simple queryset (from which it can extract
the model to hook a signal onto), and whenever something in the queryset
is saved (so, either an extra select on each save to see what items are
in the queryset, or just updating the aggregate every time - possibly
with either selectable, since the cost might be higher on either), and
it also takes the function to run on the set - either a builtin name
like "count" here, or an actual function which takes a queryset and
returns a result (in this situation the type of the underlying column
will also have to be specified).
This should probably cover most cases, and has the advantage of people
being able to specify their own in-python aggregate functions (such as
working out the standard deviation of the users' ages) if they tell us
what kind of field to sling the result in.
I'm happy to change the field names, too, CopiedField especially is a
little clunky.
Andrew
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