CODEWITHSUNDEEP
pythondjango
rtmie
rtmie

Reputation: 541

Django - Expression based model constraints

Is it possible to set an expression based constraint on a django model object, e.g. If I want to impose a constraint where an owner can have only one widget of a given type that is not in an expired state, but can have as many others as long as they are expired. Obviously I can do this by overriding the save method, but I am wondering if it can be done by setting constraints, e.g. some derivative of the unique_together constraint

 WIDGET_STATE_CHOICES = (
    ('NEW', 'NEW'),
    ('ACTIVE', 'ACTIVE'),
    ('EXPIRED', 'EXPIRED')
)

class MyWidget(models.Model):
    owner = models.CharField(max_length=64)
    widget_type = models.CharField(max_length = 10)
    widget_state = models.CharField(max_length = 10, choices = WIDGET_STATE_CHOICES)

    #I'd like to be able to do something like
    class Meta:
        unique_together = (("owner","widget_type","widget_state" != 'EXPIRED')

Upvotes: 2

Views: 1106

Answers (3)

S.Lott
S.Lott

Reputation: 391818

This is what model-based form validation is all about.

Define a form with a clean method that implements these additional rules.

Always use the form's save method to create new model objects that pass the validation rules.

http://docs.djangoproject.com/en/1.2/ref/forms/validation/#ref-forms-validation

http://docs.djangoproject.com/en/1.2/topics/forms/modelforms/#the-save-method

Upvotes: 0

Steve Jalim
Steve Jalim

Reputation: 12195

No, I don't think this will fly. The model's expecting a tuple of tuples and then the modelform base that checks it seems to grab and compare values, not run expressions.

Still, you can do it in save(), as you say - or using model validation, as DR points out

Upvotes: 0

Daniel Roseman
Daniel Roseman

Reputation: 599460

This sounds like a job for the new model validation support in Django 1.2.

Upvotes: 2

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