ondelta 0.1

A mixin that allows models to register methods that are notified when their values change, or register a method that is notified of all changes. Basically, OnDeltaMixin implements the observer pattern.

A django model mixin that makes it easy to react to field value changes on models. Supports an API similar to the model clean method.

Quick Start

Given that I have the model

class MyModel(models.Model):

mai_field = models.CharField() other_field = models.BooleanField()

And I want to be notified when mai_field’s value is changed and persisted I would simply need to modify my model to include a ondelta_mai_field method.

from ondelta.models import OnDeltaMixin

class MyModel(OnDeltaMixin):

mai_field = models.CharField() other_field = models.BooleanField()

def ondelta_mai_field(self, old_value, new_value):

print “mai field had the value of”, mai_field print “but by the time we called save it had the value of”, new_value

This is the easiest method to watch a single field for changes but what about if we want to perform an action that has an aggregate view of all of the fields that were changed? OnDeltaMixin provides an ondelta_all method for these cases which is only called once for each save.

from ondelta.models import OnDeltaMixin

class MyModel(OnDeltaMixin):

mai_field = models.CharField() other_field = models.BooleanField()

ondelta_all(self, fields_changed):

if fields_changed[‘other_field’][‘old’] == True:

print “other field was true and is now”, fields_changed[‘other_field’][‘new’] print “We also have access to”, fields_changed[‘mai_field’][‘old’]

Design Considerations

All effort to be made not to over notify on changes. Any comparison problems should fail in a way that does NOT duplication notifications. Field comparisons should also not cause other problems in the model (for example causing a child to be unable to persist).

Help

I like to help people as much as possible who are using my libraries, the easiest way to get my attention is to tweet @adamhaney or open an issue. As long as I’m able I’ll help with any issues you have.