Flapping Head

I have various models in my current Django project that can be marked as “deleted”. They’re still in the database, but filtered out and as far as most of the code is concerned, they no longer exist. A simple way to do this is with a custom manager that filters the query set.

# Custom manager filters out deleted items.
class NotDeletedManager(models.Manager):
    def get_query_set(self):
        return super(NotDeletedManager, self).get_query_set().filter(deleted=False)

class Article(models.Model):
    author = models.ForeignKey(Author, related_name='articles')
    ...
    deleted = models.BooleanField(default=False)
    # Use custom manager as default manager for this model.
    objects = NotDeletedManager()

I can then refer to Article.objects.all() and get only those articles not marked deleted. Likewise, author.articles.all() gets articles for a given author that are not marked deleted.

Next I wanted to be able to access deleted items. Time for another manager:

class DeletedManager(models.Manager):
    def get_query_set(self):
        return super(DeletedManager, self).get_query_set().filter(deleted=True)

class Article(models.Model):
    ...
    objects = NotDeletedManager()
    deleted_articles = DeletedManager()

It seems a bit redundant, but two managers is not bad. Especially since they can be used with any model that has a “deleted” field.

Next I wanted to filter related objects based on whether they belong to a deleted object. For example, if an author is deleted, all articles by that author should not show up in articles.

class NotDeletedArticleOrAuthorManager(models.Manager):
    def get_query_set(self):
        return super(NotDeletedArticleOrAuthorManager, self).get_query_set().filter(deleted=False, author__deleted=False)

class Article(models.Model):
    ...
    objects = NotDeletedArticleOrAuthorManager()

This is getting a bit messy. I’d rather pass parameters to the manager telling it what to filter out. Something like…

class FilterManager(models.Manager):
    "Custom manager filters standard query set with given args."
    def __init__(*args, **kwargs):
        self.args = args
        self.kwargs = kwargs

    def get_query_set(self):
        return super(FilterManager, self).get_query_set().filter(*self.args, **self.kwargs)

class Article(models.Model):
    ...
    objects = FilterManager(deleted=False, author__deleted=False)
    deleted_articles = FilterManager(deleted=True)

Unfortunately, this doesn’t work. The filters are applied when I call the manager directly like Article.objects.all(), but no filters are applied if I go through a relationship like author.articles.all() (returns deleted articles as well).

The problem is the related managers (see the source django/db/models/fields/related.py e.g. ForeignRelatedObjectsDescriptor.__get__). They create a class that dynamically inherits from the models default manager (our custom manager) then return a new instance of it. The new instance doesn’t have any parameters passed to its __init__ method, so there are no filters.

My workaround is to use a function to return a custom class that has the filter parameters saved in the class, so they don’t get passed to an instance. When a related manager inherits from this class, it still has our filters in place.

def get_filter_manager(*args, **kwargs):
    class FilterManager(models.Manager):
        "Custom manager filters standard query set with given args."
        def get_query_set(self):
            return super(FilterManager, self).get_query_set().filter(*args, **kwargs)
    return FilterManager()

class Article(models.Model):
    ...
    objects = get_filter_manager(deleted=False, author__deleted=False)
    deleted_articles = get_filter_manager(deleted=True)

The class is declared in the function and gets args and kwargs from the function. They are therefore “baked in” to the class and don’t need to be passed as parameters to __init__.

This is only minimally tested, but seems to work. Perhaps there is a better way, but for now this is what I have.