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

Django 1.8+ and above (Including Django 2.x and 3.x), there is a from_db classmethod, which can be used to customize model instance creation when loading from the database.

Note: There is NO additional database query if you use this method.

Here is the link to the official docs Model instance - Customize model loading

from django.db import Model

class MyClass(models.Model):
    
    @classmethod
    def from_db(cls, db, field_names, values):
        instance = super().from_db(db, field_names, values)
        
        # save original values, when model is loaded from database,
        # in a separate attribute on the model
        instance._loaded_values = dict(zip(field_names, values))
        
        return instance

So now the original values are available in the _loaded_values attribute on the model. You can access this attribute inside your save method to check if some value is being updated.

class MyClass(models.Model):
    field_1 = models.CharField(max_length=1)

    @classmethod
    def from_db(cls, db, field_names, values):
        ...
        # use code from above

    def save(self, *args, **kwargs):

        # check if a new db row is being added
        # When this happens the `_loaded_values` attribute will not be available
        if not self._state.adding:

            # check if field_1 is being updated
            if self._loaded_values['field_1'] != self.field_1:
                # do something

        super().save(*args, **kwargs)