Adding a property to a queryset in Django

Question

I'm creating a messaging system and part of what I want to do is based on whether the user is a sender or a receiver of a message, different CSS loads. I'm trying to achieve this by attaching a property to each message object that identifies this.

I have my message

#models.py
class Message(models.Model):
    def __unicode__(self):
        return unicode(self.time_date)

    message_from = models.ForeignKey(User, related_name="%(class)s_message_from")
    message_to = models.ForeignKey(User, related_name="%(class)s_message_to")
    message = models.TextField()
    file = models.FileField(blank=True, null=True)
    read = models.BooleanField(default=False)
    time_date = models.DateTimeField(default=django.utils.timezone.now)
    job = models.ForeignKey(Job, blank=True, null=True)

and my view to get the messages:

def view_messages(request):
    messages = Message.objects.filter(message_to=request.user, job_id=None).distinct('message_from')
    messages_from_user_ids = messages.values_list('message_from', flat=True).distinct()

    messages_from_user = User.objects.filter(id__in=messages_from_user_ids)

    messages = Message.objects.filter(
        Q(message_to=request.user) & Q(message_from_id=messages_from_user_ids[0]) & Q(job_id=None) | Q(
            message_from_id=messages_from_user_ids[0]) & Q(
            message_from_id=request.user) & Q(job_id=None)).order_by('time_date')

    messages = Message.objects.annotate(foo='true') # error

    return render(request, 'freelancestudent/general/messages.html', {'messages': messages,
                                                                      'messages_from': messages_from_user})

What I was trying to achieve in the line commented error was to simply test annotations by annotating the string 'true' to every message object, accessible under foo. This gives me the error 'str' object has no attribute 'resolve_expression'.

What I'm ultimately trying to achieve, in case I'm going about this the wrong way, is to check if the user is the sender. The idea i had of achieving it was to do something like this:

for message in messages:
    if message_from == request.user.id:
        messages.annotate(sender=True)

Though, this isn't the right syntax. I'm not sure annotate is the right function, either. Any ideas?

That sounds more like a template design thing.. Do you not need to check there if the message sender is the one looking at it? — Sayse
– Sayse, Commented Jan 27, 2016 at 17:42

Alasdair · Accepted Answer · 2016-01-27 17:53:24Z

12

If you are looping through the queryset, don't try to use annotate, just set an attribute on the instance.

for message in messages:
    if message_from == request.user.id:
        message.sender = True

Alternatively, I think you could use annotate with Case:

from django.db.models import BooleanField, Case, Value, When

messages = Message.annotate(
    sender=Case(
        When(message_from=request.user, then=Value(True)),
        default=Value(False),
        output_field=BooleanField(),
    )
)

edited Jan 27, 2016 at 17:53

answered Jan 27, 2016 at 17:47

Alasdair

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2 Comments

pygeek Over a year ago

Note that support for conditional expressions is introduced in Django 1.9. docs.djangoproject.com/en/1.9/ref/models/…

Alasdair Over a year ago

@pygeek I thought they were new in 1.8, which is now the minimum supported version.

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