Filter a contenttype with list of content object's field

Question

I creating an app where users can post with its related tags:

class Tag(models.Model):
    name = models.CharField(max_length=255, unique=True)

class Post(models.Model):
    user = models.ForeignKey(User)
    body = models.TextField()
    tags = models.ManyToManyField(Tag)
    pub_date = models.DateTimeField(default=timezone.now)
    activity = GenericRelation(Activity, related_query_name="posts")

class Photo(models.Model):
    user = models.ForeignKey(User)
    file = models.ImageField()
    tags = models.ManyToManyField(Tag)
    pub_date = models.DateTimeField(default=timezone.now)
    activity = GenericRelation(Activity, related_query_name="photos")

class Activity(models.Model):
    actor = models.ForeignKey(User)
    verb = models.PositiveIntegerField(choices=VERB_TYPE)
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('content_type', 'object_id')
    pub_date = models.DateTimeField(default=timezone.now)

What I want to do is get/filter maximum of 5 latest/recent Activity objects, with the a list of users, and a list of tags from the list of Post objects tags field and return json using django-rest-framework to view in the client side.

Eg activities:

• UserA created a new Post obect with tags(#class, #school)
• UserB created a new Post object with tags(#professor, #teacher)
• UserC created a new Post object with tags(#school, #university)
• UserD created a new Post object with tags(#university, #school)

So say I want to filter Activity with user_list=[UserA, UserC] and tag_list = [#class, #teacher]

It should return:

• UserA created a new Post obect with tags(#class, #school)
• UserC created a new Post object with tags(#school, #university)
• UserB created a new Post object with tags(#professor, #teacher)

To filter the Activity with users, I can query this way:

Activity.objects.filter(actor__in=user_list)

But, how do I filter Activity with the content_object's (i.e.Post or Photo) field (i.e. Post.tags or Photo.tags)? Now I am doing this way:

Activity.objects.filter(posts__tags__in=tag_l)
Activity.objects.filter(photos__tags__in=tags)

So to sum up, If I have need activities with list of users and list of tags I have to do like this:

activites = Activity.objects.filter(
    Q(actor__in=user_list) |
    Qposts__tags__in=tag_list) |
    Q(photos__tags__in=tag_list)
)

But suppose there will be more than two ContentType model classes then I'd have to again add another Q(moreModel__tags__in=tag_list). So, I hope there's a better way to optimize the process.

Answer 1

I'd say your best bet would be to use a filter from Django filter (link to the rest framework docs), specifically a ModelMultipleChoiceFilter. I'm going to assume you already have an ActivityViewSet to go along with the Activity model.

Firstly you'll want to create a django_filters.FilterSet, probably in a new file such as filters.py, and set up the ModelMultipleChoiceFilter, like so:

import django_filters

from .models import Activity, Tag, User

class ActivityFilterSet(django_filters.FilterSet):
    tags = django_filters.ModelMultipleChoiceFilter(
        name='content_object__tags__name',
        to_field_name='name',
        lookup_type='in',
        queryset=Tag.objects.all()
    )
    users = django_filters.ModelMultipleChoiceFilter(
        name='content_object__user__pk',
        to_field_name='pk',
        lookup_type='in',
        queryset=User.objects.all()
    )

    class Meta:
        model = Activity
        fields = (
            'tags',
            'users',
        )

Then you'll want to tell your viewset to use that filterset:

from .filters import ActivityFilterSet
# ...
class ActivityViewSet(GenericViewSet):
    # all your existing declarations, eg.,
    # serializer_class = ActivitySerializer
    # ...
    filter_class = ActivityFilterSet
    # ...

Once you've got that done, you'll be able to filter your results using GET parameters, eg.,

• GET /activities?users=1 – everything created by User 1
• GET /activities?users=1&users=2 – everything created by User 1 or User 2
• GET /activities?users=1&tags=class – everything created by User 1 with the tag #Class
• GET /activities?users=1&users=2&tags=class&tags=school – everything created by User 1 or User 2 with the tags #Class or #School
• and so on