Generic relations vs. iterating on objects in Django - django

Question

I have three models called Post, Category and Flag.
I let users to make a Flag on each Post which is tagged with a Category. Now I want to list all of Flags, related to Posts on a certain Category.
I've found two ways of doing it using ListView generic view:
First approach:
Define a generic relation between Flag and Category so I can call something like: Flag.objects.filter(object__categor=self.category)
Second approach:
def get_queryset(self):
pk = self.kwargs['pk']
self.category = get_object_or_404(Category, pk=pk)
flags = Flag.objects.all()
qs=[]
for f in flags:
if f.object.category == self.category:
qs.append(f)
return qs
My question is which approach is more efficient? I'm not familiar with generic relations, but it seems a little bit heavy to do. On the other hand, querying on all Flag objects and iterating through them isn't cheap one.
Models.py
class Flag(models.Model):
flagger = models.ForeignKey(User, related_name='flaggers')
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
object = GenericForeignKey('content_type', 'object_id')
submit_date = models.DateTimeField(auto_now_add=True)
reason = models.IntegerField(choices=REASON_CHOICES, default=BROKEN_LINK)
class Category(models.Model):
name = models.CharField(max_length=100)
slug = models.SlugField(unique=True)
...
class Base(models.Model):
title = models.CharField(max_length=200)
slug = models.CharField(max_length=200, db_index=True)
category = models.ForeignKey(Category, default=1)
...
class Text(Base):
description = models.TextField(max_length=500)
...
class Image(Base):
image = models.ImageField()
class Post(models.Model):
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
created_at = models.DateTimeField(null=True)
After saving each of Text or Image instances, I create a Post model. So I can get either Posts flagged or text flagged and image flagged separately.
[also any tip about how can I define generic relation in case I should use it, would be appreciated]