django - comparing old and new field value before saving

Question

I have a django model, and I need to compare old and new values of field BEFORE saving.

I\'ve tried the save() inheritence, and pre_save signal. It was triggered cor

Answer 1

Here is an app that gives you access to previous and current value of a field right before model will be saved: django-smartfields

Here is how this problem can be solved in a nice declarative may:

from django.db import models
from smartfields import fields, processors
from smartfields.dependencies import Dependency

class ConditionalProcessor(processors.BaseProcessor):

    def process(self, value, stashed_value=None, **kwargs):
        if value != stashed_value:
            # do any necessary modifications to new value
            value = ... 
        return value

class MyModel(models.Model):
    my_field = fields.CharField(max_length=10, dependencies=[
        Dependency(processor=ConditionalProcessor())
    ])

Moreover, this processor will be invoked, only in case that field's value was replaced

Answer 2

I agree with Sahil that it is better and easier to do this with ModelForm. However, you would customize the ModelForm's clean method and perform validation there. In my case, I wanted to prevent updates to a model's instance if a field on the model is set.

My code looked like this:

from django.forms import ModelForm

class ExampleForm(ModelForm):
    def clean(self):
        cleaned_data = super(ExampleForm, self).clean()
        if self.instance.field:
            raise Exception
        return cleaned_data