Sharing validation with the admin under DRF 3

[Ben Warren]

I have a question about writing reusable validation logic shared between the admin and DRF. If I have this setup: 

class Foo(models.Model):
  from = models.IntegerField()
  to = models.IntegerField()
  def clean(self): if self.from > self.to:
    raise ValidationError("`From` must be before or equal to `to`")

class FooSerializer(serializers.ModelSerializer):
  class Meta:
    model = Foo

Then starting with DRF3 the clean() validation will not be called by the serializer. The docs mention that I could use this mixin: 

def validate(self, attrs):
  model = self.Meta.model
  instance = Model(attrs)
  instance.clean()

but that it is dispreferred, and suggests "you really should look at properly separating the validation logic out of the model method if possible". How would I factor logic like the above so that it could be used in the admin and in DRF? What if the logic gets more complex, like "field B can be populated only if field A is > 7"?

Would I do something like this?  The reach into __dict__ feels weird.

def ordervalidator(dct):
   if self.from > self.to:
      raise ValidationError # Using Django ValidationError, because the model admin needs it?

class Foo(models.Model):
  from = models.IntegerField()
  to = models.IntegerField()
  validators = [ordervalidator]
  def clean(self):
   for validator in self.validators:
      validator(self.__dict__)    

class FooSerializer(serializers.ModelSerializer): 
  class Meta: 
    model = Foo 
    validators = [ordervalidator]

[Brent O'Connor]

I think all you need to do is add the following to your model.

class Foo(models.Model):
  from = models.IntegerField()
  to = models.IntegerField()

  def clean(self): if self.from > self.to:
      raise ValidationError("`From` must be before or equal to `to`")

  def save(self, **kwargs):
      self.full_clean()
      super().save(**kwargs)

[Carl Nobile]

What @Brent said is basically correct, however, it will fail if you have model mixins that change fields. That's because the field will not be changed until after the full_clean() method is called.
So how to fix this? It's not too difficult. I create a mixin that just does the full_clean() as in the following:

class ValidateOnSaveMixin:     def save(self, *args, **kwargs):         self.full_clean()         super().save(*args, **kwargs)

In your class you would do this:

class Foo(ValidateOnSaveMixin, models.Model):

    from = models.IntegerField()
    to = models.IntegerField()
    validators = [ordervalidator]
  

    def clean(self):
        for validator in self.validators:
            validator(self.__dict__)

Always be sure you put the ValidateOnSaveMixin class is just before the models.Model and all other mixins before ValidateOnSaveMixin. Getting the MRO correct is important here.

~Carl

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