Django signals for changed fields
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fieldsignals
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README.md

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Introduction

django-fieldsignals simply makes it easy to tell when the fields on your model have changed.

Often model updates are quite expensive. Sometimes the expensive operations are very rare. This makes it tempting to put the update logic in a view, rather than in a save() method or in a signal receiver:

    # A bad example. Don't do this!
    def edit_poll(request, poll_id):

        # ...

        if form.cleaned_data['poll_name'] != poll.name:
            poll.update_slug(form.cleaned_data['poll_name'])
        poll.save()

That's a bad idea, because your model consistency is now dependent on your view.

Instead, use django-fieldsignals:

    from fieldsignals import pre_save_changed

    def update_poll_slug(sender, instance, **kwargs):
        instance.slug = slugify(instance.name)

    pre_save_changed.connect(update_poll_slug, sender=Poll, fields=['name'])

In case you want to know what changed, django-fieldsignals even tells you the old and new values of your fields:

    from fieldsignals import pre_save_changed

    def print_all_field_changes(sender, instance, changed_fields=None, **kwargs):
        for field, (old, new) in changed_fields.items():
            print "%s changed from %s to %s" % (field.name, old, new)

    pre_save_changed.connect(print_all_field_changes, sender=Poll)

Installation

  1. This library is on PyPI so you can install it with:
    pip install django-fieldsignals

or from github:

    pip install 'git+https://github.com/craigds/django-fieldsignals.git#egg=django-fieldsignals'
  1. Add "fieldsignals" to your INSTALLED_APPS setting like this:
    INSTALLED_APPS = (
        ...
        'fieldsignals',
    )
  1. Add some signals!

Where should my signals code live?

Field signals must be connected after the django apps are ready. So putting signal connectors at the bottom of your models file, or other random places won't work.

The best place to connect fieldsignals is an AppConfig.ready() handler.

Notes

  • Currently no support for ManyToManyField or reverse side of ForeignKey (one to many).