As you know, model managers can be overriden in Django. It's convenient to add custom filtration method there:
But these custom methods cannot be chained:
Overriding manager doesn't allow it, because after first manager method is invoked (
publishedin this case), we receive queryset as a result, which knows nothing about custom manager methods. So we have to add custom methods to queryset. This can look like this:
class ArticleQuerySet(models.query.QuerySet): def published(self): return self.filter("...") def old(self): return self.filter("...")
Now we need to make manager user this class. In order to do this, let's override manager's
class ArticleManager(models.Manager): def get_query_set(self): return ArticleQuerySet(self.model, using=self._db) class Article(models.Model): # ... objects = ArticleManager()
This allows to make queties like:
Here we invoke
all to get queryset instance, because this time manager doesn't have custom methods -
old. To avoid this, we can make manager look for undefined methods in queryset:
class ArticleManager(models.Manager): def get_query_set(self): return ArticleQuerySet(self.model, using=self._db) def getattr(self, key): return getattr(self.get_query_set(), key)
getattris only invoked when attribute is not found by usual means.
Manager can be further improved to be more generic:
class QuerySetManager(models.Manager): def init(self, queryset_class, args, **kwargs): self.queryset_class = queryset_class super(QuerySetManager, self).init(args, **kwargs) def get_query_set(self): return self.queryset_class(self.model, using=self._db) def getattr(self, key): return getattr(self.get_query_set(), key) # Теперь можно писать просто: # objects = QuerySetManager(ArticleQuerySet) #
In my current project I had a task to use twitter API. Twitter uses OAuth for authentication, which is pretty dreary. To avoid fiddling with it all the time, I've moved authentication to decorator. If key is available - nothing happens, just view is launched as usual. It's convenient that there's no need for additional twitter settings in user profile. Code is in article.