I am trying to write my first django app and am unclear about good
ways to deal with many to many relationships with through tables. My
app is like a bibliography but with lots of links between entries in
the bibliography. For example, I want to store links to all the
entries cited by other entries. There are thirteen types of relations
between entries that I want to store so I am using a type field in the
through table for that. Some greatly simplified code is below. This
code is working but it seems to me that in the view, i.e. the index
function, that it will hit the database a lot more than is really
necessary. I suspect there are much better ways of handling it if only
I understood django better hence this question to you. I was looking
at annotate but this isn't really aggregation so it doesn't seem like
I can use it. Does any one have any helpful advice?
class Resource(models.Model):
# entry in the bibliograpy or something related to an entry
title = models.CharField(max_length=255)
year = models.IntegerField(null=True, blank=True)
publisher = models.ForeignKey(Publisher, null=True, blank=True)
is_resource = models.BooleanField(default=False)
relations = models.ManyToManyField('self',
through='ResourceRelationship', symmetrical=False)
def __unicode__(self):
return self.title
def get_container(self):
try:
cont = ResourceRelationship.objects.get
(from_resource=self.id,
relationship=ResourceRelationship.IS_PART_OF).to_resource
except ResourceRelationship.DoesNotExist:
cont = None
return cont
class ResourceRelationship(models.Model):
relationship = models.IntegerField(choices=RELATIONSHIP_CHOICES)
from_resource = models.ForeignKey(Resource,
related_name='from_resources')
to_resource = models.ForeignKey(Resource,
related_name='to_resources')
order = models.PositiveSmallIntegerField(default=1)
def index(request):
resource_list = Resource.objects.filter(is_resource=True)
for resource in resource_list:
container = resource.container = resource.get_container()
if container:
resource.year = container.year
resource.publisher = container.publisher
return render_to_response('resources/index.html',
{'resource_list': resource_list})
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