Generic relations vs. iterating on objects in 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]