Django query annotate after filter doesn't work correctly

Question

I have two models that are related using a m2m relationship through an intermediate model:

class Chat(models.Model):
    participants = models.ManyToManyField(settings.AUTH_USER_MODEL,
                                          through="Join", blank=True,
                                          related_name="chats")


class Join(models.Model):
    """Intermediate model for relations between a user and a chat."""

    chat = models.ForeignKey(Chat, on_delete=models.CASCADE, related_name="+")
    user = models.ForeignKey(settings.AUTH_USER_MODEL,
                             on_delete=models.CASCADE,
                             related_name="joins")
    date_joined = models.DateField(auto_now_add=True)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=("user", "chat"),
                name="user_join_once_constraint"
            )
        ]

When I do this:

chats = Chat.objects.annotate(Count("participants")).filter(participants=user)
chats.get(pk=11).participants__count

It will return 2; which is correct. But when I do this:

chats = Chat.objects.filter(participants=user).annotate(Count("participants"))
chats.get(pk=11).participants__count

It will return 1.

My understanding is that the first one is counting the number of relations for each chat object while the second one is counting how many times the user is related to each chat object.

What I want is to annotate a pre-evaluated queryset with number of relationships for each object.

What's wrong? what am I doing wrong? I need to annotate a filtered queryset.

---

SQL:

First one (annotate first):

SELECT "msgr_chat"."id", "msgr_chat"."lat", COUNT("msgr_join"."user_id") AS "participants__count" FROM "msgr_chat" LEFT OUTER JOIN "msgr_join" ON ("msgr_chat"."id" = "msgr_join"."chat_id") INNER JOIN "msgr_join" T4 ON ("msgr_chat"."id" = T4."chat_id") WHERE T4."user_id" = 7 GROUP BY "msgr_chat"."id"

Second one (filter first):

SELECT "msgr_chat"."id", "msgr_chat"."lat", COUNT("msgr_join"."user_id") AS "participants__count" FROM "msgr_chat" INNER JOIN "msgr_join" ON ("msgr_chat"."id" = "msgr_join"."chat_id") WHERE "msgr_join"."user_id" = 7 GROUP BY "msgr_chat"."id"

Answer 1

I found the answer to this problem in Django docs.

Apparently:

filter() and annotate() are not commutative operations.

I have to annotate all of the objects first, and then use filter on it; to almost get what I want.

But I still don't know what should I do if I want to annotate a queryset that is not directly from the model's manager.