Creating efficient database queries for hierarchical models (django)

Question

Consider this (django) model:

class Source(models.Model):
   # Some other fields
   type = models.ForeignKey(\'Type\')

class Type(models.Model):
    # Some othe         


        

Answer 1

How do I efficiently query so that if I select Source by Type, I also retrieve Sources which have a Type that is a child of the given type?

For the example given, it is fairly easy to setup a query because no recursive calls need to be made, and your "hierarchy" is only one level deep:

class Source(models.Model):
   # Some other fields
   type = models.ForeignKey('Type')

class Type(models.Model):
    # Some other fields
    name = models.CharField(max_length=100)
    parent = models.ForeignKey('self', blank=True, null=True)

#We want all sources under in the type = Radio tree
type = Type.objects.get(name='Radio')
qs = Source.objects.filter(type__parent=type)

#We want all sources that are `Forum` typed
type = Type.objects.get(name='Forum')
qs = Source.objects.filter(type=type)

This is assuming that Source is always related to a "child" type and not to the "parent."

If sources can also be related to the "parent" types, you can use Q for complex queries:

>>> from django.db.models import Q
>>> type = Type.objects.get(name='Radio')
>>> qs = Source.objects.filter(Q(type=type)|Q(type_parent=type))
>>> #if Radio type id = 2
>>> print qs.query
SELECT `app_source`.`id`, `app_source`.`type_id` FROM `app_source` INNER JOIN `app_type` ON  (`app_source`.`type_id` = `app_type`.`id`) WHERE (`app_source`.`type_id` = 2  OR `app_type`.`parent_id` = 2 )
>>> 

If you have a truly hierarchical trees in your tables, this method is much less usable, and you should seek out another solution.