django: moving business rules from views and forms to model

Since django 1.2 it is able to write validation code on model. When we work with modelforms, instance.full_clean() is called on form validation. But this code is not execute on save() method:

Note that full_clean() will not be called automatically when you call your model’s save() method, nor as a result of ModelForm validation. 

For this reason, I alltimes call clean() model method before save model. I invoke this call trhough pre_save signal.

For more business rules issolation, I write all them in a separate python module: rules.py.

This is models.py ---------------------------------------------------------------

from django.db import models

class Issue(models.Model):
    ....

def clean(self):

rules.Issue_clean(self)

from issues import rules

rules.connect()

This is rules.py ---------------------------------------------------------------

from issues.models import Issue

def connect():

from django.db.models.signals import post_save, pre_save, pre_delete

#issues

pre_delete.connect(Issue_pre_delete, sender= Incidencia)

pre_save.connect(Issue_pre_save, sender = Incidencia )

post_save.connect(Issue_post_save, sender = Incidencia )

def Incidencia_clean( instance ):

import datetime as dt

errors = {}

#dia i hora sempre informats

if not instance.dia_incidencia:

errors.setdefault('dia_incidencia',[]).append(u'Falten Dades: Cal dia i franja')

#dia i hora sempre informats

if not  instance.franja_incidencia:

errors.setdefault('franja_incidencia',[]).append(u'Falten Dades: Cal dia i franja')

#Només es poden posar incidències més ennlà de 7 dies

if instance.dia_incidencia < ( dt.date.today() + dt.timedelta( days = -7) ):

errors.setdefault('dia_incidencia 1',[]).append(u'''No es poden posar o modificar incidències amb més d' una setmana)''')

#No incidències al futur.

if instance.getDate() > datetime.now():

errors.setdefault('dia_incidencia 2',[]).append(u'''Encara no pots posar incidències en aquesta classe. Encara no s'ha realitzat.''')

#No incidencies alumne que és baixa:

if instance.alumne.data_baixa is not None and instance.alumne.data_baixa < instance.dia_incidencia:

errors.setdefault('dia_incidencia 3',[]).append(u'''L'alumne estava de baixa en aquesta data.''')

if len( errors ) > 0:

raise ValidationError(errors)

def Issue_pre_save(sender, instance, **kwargs):

instance.clean()

In order to show errors on form, you should include this on form template:

{% if form.non_field_errors %}

{% for error in form.non_field_errors %}

{{error}}

{% endfor %}

{% endif %}   The reason is that model validation erros ara binded to non_field_errors error dictionary entry.

When you save or delete a model out of a form:

    try:
        issue.delete()
    except ValidationError, e:
        import itertools
        errors = list( itertools.chain( *e.message_dict.values() ) )

To add errors to a form dictionary on no modelforms:

        try:
            #provoco els errors per mostrar-los igualment al formulari.
            issue.clean()
        except ValidationError, e:
            form._errors = {}
            for _, v in e.message_dict.items():
                form._errors.setdefault(NON_FIELD_ERRORS, []).extend(  v  )

You can see samples on ghap project source code.

Here some samples: