On Thu, 2009-05-07 at 07:12 -0700, Aldo wrote:
> Hello,
>
> I have a problem for a quite simple model.
> I would like to model a person marriage relation.
>
> example :
>
> class Person(models.Model):
> name=models.CharField(max_length=100)
> married_to=models.OneToOneField('self',symmetrical=True) #
> symmetrical not allowed
> married_to=models.ForeignKey
> ('self',unique=True,symmetrical=True) # symmetrical not allowed
This is because "symmetrical" requires two entries (or making two
filters every time you look something up). Only many-to-many fields can
be symmetrical, because they have an entry for the pair (A, B) and
another entry for (B, A).
> married_to=models.ManyToManyField
> ('self',unique=True,symmetrical=True) # unique not allowed
It's called many-to-many. Multiple entries, pretty much by definition.
Also, as noted, symmetrical and unique can't work together on an
implementation level.
>
> The relation I want has to be unique and symmetrical.
> Someone can only be married to one other person (unique).
> If Alice is married to Bob, then Bob is automatically married to Alice
> (symmetrical).
>
> How would you model this relation
There are many relations between data that simply cannot be modelled at
the model level. Even modelling them at the database level isn't always
possible.
In this case, you get to choose which way you want to model things at
the Model subclass level and then enforce the remaining portions in,
say, the save() method or in the forms you're using to populate the
model. You could choose to have a nullable OneToOneKey between Person
objects and remember to check both whether the relation or the reverse
relation exists when trying to add a new one. Or your could use a
ManyToManyField and check that no entry already exists when adding a new
one. In either case, some extra checking will be required by you. That's
simple because this constraint is sufficiently complex that it's not
covered by default (even writing an SQL-level table constraint would
require a table lookup on every insert, for example).
Regards,
Malcolm
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