postgres partial index for django

models.py

class PartialIndex(Index):
    def __init__(self, fields=[], name=None, condition=None, unique=False):
        # TODO accept condition as dictionary and make sql escaping and quoting
        self.condition = condition
        self.unique = unique
        super(PartialIndex, self).__init__(fields=fields, name=name)
 
    def deconstruct(self):
        path, args, kwargs = super(PartialIndex, self).deconstruct()
        kwargs['unique'] = self.unique
        if self.condition is not None:
            kwargs['condition'] = self.condition
        return path, args, kwargs
 
    def get_sql_create_template_values(self, model, schema_editor, using):
        parameters = super(PartialIndex, self).get_sql_create_template_values(model, schema_editor, using=using)
        if self.condition is not None:
            parameters['extra'] = ' WHERE ({})'.format(self.condition) + parameters['extra']
        return parameters
 
    def create_sql(self, model, schema_editor, using=''):
        if self.unique:
            sql_create_index = schema_editor.sql_create_index.replace("INDEX", "UNIQUE INDEX", 1)
        else:
            sql_create_index = schema_editor.sql_create_index
 
        sql_parameters = self.get_sql_create_template_values(model, schema_editor, using)
        return sql_create_index % sql_parameters

class PaymentRequest(models.Model):
    PAYMENT_REQUEST_STATUSES = Choices(
        ('OPEN', _('open')),
        ('CANCELED', _('canceled')),
        ('PAYED', _('payed')),
    )
    order = models.ForeignKey('order.Order', related_name='payment_requests')
    status = models.CharField(max_length=15, choices=PAYMENT_REQUEST_STATUSES)
    amount = models.DecimalField(max_digits=12, decimal_places=2)
 
    def __unicode__(self):
        return "{status} payment request of amount {amount} for order {order_id}".format(
            status=self.status,
            amount=self.amount,
            order_id=self.order_id,
        )
 
    class Meta:
        indexes = [
            PartialIndex(name='unique_open_request_per_order',
                         fields=['order_id'], unique=True, condition="status='OPEN'", )
        ]