Django相关model字段查询（互交朋友）

Question

I have a friendship model:

class Friendship(models.Model):
    user = models.ForeignKey(
        Account, on_delete=models.CASCADE, related_name="friend1", null=True, blank=True)
    other_user = models.ForeignKey(
        Account, on_delete=models.CASCADE, related_name="friend2", null=True, blank=True)
    date_created = models.DateTimeField(auto_now=True)

    objects = FriendshipManager()

    class Meta:
        verbose_name = "friendship"
        verbose_name_plural = "friendships"
        unique_together = ("user", "other_user")

    def __str__(self):
        return f'{self.user} is friends with {self.other_user}.'

and this function to return all users who are mutual friends of two accounts

def mutual_friends(self, account1, account2):
        mutual_friends = Account.objects.filter(
            Q(friend2__user=account1) & Q(friend2__user=account2))
        return mutual_friends

Based on my (limited) understanding of how the query api works, I would think this should return all users who have a "friend2" relationship with the Friendship table where the "friend1" user is either account1 or account2. I'm still getting used to querying with django, so if someone can let me know what I'm doing wrong that'd be great.

Thanks!

Answer 1

Your model design doesn't seem right to me. As of now, you can put any Account instance as user or other_user , and as they both refer the same model ( Account ), while doing any retrieval from database, you need to account for both of the fields.

A better design IMO would be to use a ManyToManyField (many-to-many relationship) in the Account model to itself, as one account can have multiple other accounts as friends and vice-versa. So:

class Account(models.Model):
    ...
    friends = models.ManyToManyField('self')
    ...

Now, you can add friends like eg:

account_foo.friends.add(account_bar, account_spam)

account_* are Account instances.

You can get all friends of account_foo like:

account_foo.friends.all()

Check out the many-to-many doc for various examples of data integration and querying.

---

Now, to find the mutual friends of eg account_foo and account_bar , you can first get all the friends of account_foo and then see which of them are also friends with account_bar :

friends_with_foo = account_foo.friends.values_list('pk', flat=True)
mutual_friends_of_foo_bar = account_bar.friends.filter(pk__in=friends_with_foo)