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 (
published in 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)
getattr is 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) # Now you can simply write: # objects = QuerySetManager(ArticleQuerySet)
It appears that not everyone knows that in python you can create classes dynamically without metaclasses. I'll show an example of how to do it.So we've learned how to use custom QuerySet to chain requests:Article.objects.old().public()Now we need to make it work for related objects:user.articles.old().public()This is done using use_for_related_fields, but it needs a little trick.
In one of our recent projects, we had an interesting case: the whole application was built around an interactive map for a fairly large shopping mall, and the main goal of the system was to plot the closest route to a user-selected destination.