Validating a Django model field based on another field's value?

Question

I have a Django app with models accessible by both Django REST Framework and a regular form interface. The form interface has some validation checks before saving changes to the model, but not using any special Django framework, just a simple local change in the view.

I'd like to apply the same validation to forms and REST calls, so I want to move my validation into the model. I can see how to do that for simple cases using the validators field of the Field, but in one case I have a name/type/value model where the acceptable values for 'value' change depending on which type is selected. The validator doesn't get sent any information about the model that the field is in, so it doesn't have access to other fields.

How can I perform this validation, without having essentially the same code in a serializer for DRF and my POST view for the form?

Answer 1

The validation per-field doesn't get sent any information about other fields, when it is defined like this:

def validate_myfield(self, value):
    ...

However, if you have a method defined like this:

def validate(self, data):
    ...

Then you get all the data in a dict, and you can do cross-field validation.

Answer 2

I dug around codebase of drf a little bit. You can get values of all fields using following approach. Doing so, you can throw serialization error as {'my_field':'error message} instead of {'non_field_error':'error message'}.

def validate_myfield(self, value):
   data = self.get_initial() # data for all the fields
   #do your validation

However, if you wish to do it for ListSerializer, i.e for serializer = serializer_class(many=True), this won't work. You will get list of empty values. In that scenario, you could write your validations in def validate function and to avoid non_field_errors in your serialization error, you can raise ValidationError with error message as a dictionary instead of string.

def validate(self, data):
    # do your validation
    raise serializers.ValidationError({"your_field": "error_message"})