The UpdateFormMixin is a mixin for a WebAPIResource that allows a

resource to specify a form class (which should be a subclass of

django.forms.ModelForm) to use for creating and updating of model

instances. This allows the resource that are very basic wrappers around

models to have their logic further simplified.
This mixin is required for allowing partial updating of models.

Traditionally, a ModelForm can take an instance parameter

specifying an already existing model instance to update, but will still

require all required field to specified in the form data. This mixin

takes care of that by extracting missing form data from the instance

and inserting it into the form data in the proper format (such as

setting it to be a list of primary keys in the case of a

ModelMultipleChoiceField).
This mixin also allows resources to specify special functions for

parsing values out of the recieved form data. This is useful, e.g., for

automatically converting human-readable values accepted by the API to

the value the database expects for a ChoiceField.

Used this in a Review Board API resource.