django-lfs / docs / developer / howtos / how_to_add_own_criteria / index.rst

How to add an own criteria

In this how-to you will learn how to add an own criterion.

The goal is to create a criterion, in which the customer can enter a SKU and decide (via operators) whether the criterion is valid if the product, with the entered SKU, is in the cart or not.

Please see also the example application :download:`product_criterion <product_criterion.tar.gz>` or refer to the default implementation of LFS within lfs.criteria.models.

Create an application

First you need to create a default Django application (or use an existing one), where you can put in your plugin. If you do not know how to do this, please refer to the excellent Django tutorial.

Implement the Criterion Model

The main part of the criterion consists of a model which must inherit from the Criterion base class.

Create the Class

class ProductCriterion(Criterion):
    value = models.CharField(max_length=100)

The only attribute we need is the value the shop admin will save for the criterion. The attribute can have any type you need. In this example we use a simple character field. This example criterion will be valid, when the the product with the given SKU is within the card.

Implement necessary Methods

In the next steps we implement all necessary methods which are needed to make the criterion work.

The get_operators method needs to return the available operators for this criterion. It is a list of list, whereas the first value is an integer and the second value is the name of the operator.

def get_operators(self):
    return [
        [0, _(u"Is in cart")],
        [1, _(u"Is not in cart")],

The is_valid method needs to return a boolean. If it returns True the criterion is considered valid, it returns False the criterion is considered not valid.

def is_valid(self):
    if self.product:
        return self.value == self.product.sku
    elif self.cart:
        result = any([self.value == item.product.sku for item in self.cart.get_items()])
        return result if self.operator == 0 else not result
        return False


Within the is_valid method (as in all methods of the Criterion class) following attributes are available:

This is only set, when the criterion is called from the product detail view otherwise it is None.
The current cart of the current user.
The current request.

Please see also the example application :download:`product_criterion <product_criterion.tar.gz>` for the complete implementation of the model.

Plug in the Criterion

Now as the code is ready, you can easily plugin your own criterion:

  1. Add your application to the PYTHONPATH

  2. Add your application to settings.INSTALLED_APPS and sync your database:

  3. Add the class to the :ref:`LFS_CRITERIA <settings_lfs_criteria>` setting:

        ["product_criterion.models.CartPriceCriterion", _(u"Product Criterion")],
  4. As all criteria are models, you have to synchronize your database:

    $ bin/django syncdb
  5. Restart your instance and the criterion should be available for selection, for instance within the discount criteria tab.

And that's it

You should now see your criterion within the criteria tab of Discounts for instance. You can enter a product SKU to it and select one of the above mentioned operators.

Good to know

  • You can also create criteria with select or multiple select fields. See the :ref:`API <api_criterion>` or the default Country criterion within lfs.criteria.models for more.
  • You can override more than the two mentioned methods above. See the :ref:`Criterion API <api_criterion>` which methods are provided by the base class.

See Also