On Wed, 16 Mar 2011 07:49:17 +0200, Michal Petrucha
<michal.petru...@ksp.sk> wrote:
On Tue, Mar 15, 2011 at 09:15:29PM -0500, Javier Guerra Giraldez wrote:
On Tue, Mar 15, 2011 at 7:14 PM, Christophe Pettus <x...@thebuild.com>
wrote:
> A concern here is that composite indexes, like unique, are
> sensitive to the ordering of the fields, which means that the
> ordering of the fields in the class declaration becomes important.
This is the same reason a new Meta flag has been agreed upon in the
past. (That, however, does not mean it has to be that way.)
a simplistic proposal:
the order of the fields on a composite index is determined by the
exact value given to the primary_key argument.
that way, just setting primary_key=True on a few fields won't
guarantee order, but something like:
class City (models.Model):
country = models.CharField (max_length=2, primary_key=1)
state = models.CharField (max_length=2, primary_key=2)
city = models.CharField (max_length=3, primary_key=2)
Name = models.CharField (max_length=20)
would set the (country,state,city) primary key in the obvious order.
in short: giving any non-falsy value to primary_key would add the
field to the key, the exact value would determine ordering.
I like this proposal. It might even be easier to implement than
fiddling with new Meta flags and everything.
One minor detail though, just setting primary_key=True on multiple
fields would still have to guarantee some ordering since the primary
key would be represented by a tuple. If you don't know for sure in
which order the values are, you can't really use the .pk property.
This feels like something that wants a named-tuple (or a full blown dict).
Alternatively, provide a method on the model class that takes the
name=value arguments (queryset style) and returns the right pk tuple.
Otherwise, the exact ordering of the fields in the pk tuple becomes yet
another implicit(!) part of the model's contract - and any code that wants
to use this model will be that much more brittle.
This would require a thorough explanation in the docs, that if you
supply values that compare equal to primary_key, the order of fields
will be the same as in the model definition.
Michal Petrucha
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