Django: combine items from two queries (two different Models) together - mysql

Question

I'm building an event platform with Django with some events public and some invite-only. Here's how it basically works:
Every event is based on the Event model.
Private events are only viewable by invited guests: this is achieved
through an Invite model referencing both the event and the guest.
Users can inform whether they will be attending an event or not
through an Answer model that stores the associated event, user
and the answer.
The Answer and Invite models are totally independent from each other, that way I am able to use the Answer model for both public and private events.
What I'm trying to achieve:
For each event where I'm invited, display the invitation (event.creator invited you to event.name) and, if it exists, my associated answer, else display a form to answer.
So I think what I'm trying to do is getting all events where I'm invited (I have no problem with that) and joining that to my answer (where user=me). The problem is that the answer might not even exist yet --if I haven't answered.
Ideally, I would have all of that in one single query so that I could do something like this in the templates: invited_event.answer and display the answer if it exists.
EDIT:
So I think what I need ultimately is to mix two queries: one that gets all the events where I'm invited (1) and an other that grabs all answers for those events (2).
(1)
latest_events_invited = Event.objects.filter(invite__user_invited=request.user)
(2)
answers_events_invited = Answer.objects.filter(user_answering=request.user, event__in=latest_events_invited)
Something like: for each event in latest_events_invited, append corresponding answer from answers_events_invited.
Any ideas?
Code:
My template (index.html):
<h3>Invites</h3>
{% if latest_invites_list %}
<ul>
{% for event in latest_events_invited %}
<li>
{{ event.creator }} invited you to {{ event }}<br/ >
<!--IDEALLY:-->
{% if event.answer %}
You answered: {{ answer.answer }}
{% else %}
<form action="{% url 'events:answer_event' invite.event.id %}" method="post">
{% csrf_token %}
Answer:
<select name="answer">
<option value="1" >Attending</option>
<option value="2" >Not attending</option>
</select>
<input type="submit" name="submit" value="Answer">
</form>
{% endif %}
</li>
</ul>
{% else %}
<p>No invites.</p>
{% endif %}
The view (views.py)
def index(request):
if request.user.is_authenticated():
latest_events_invited = Event.objects.filter(invite__user_invited=request.user)
latest_answers_list = Answer.objects.filter(user_answering=request.user, event__in=latest_events_invited)
#do something with those to get: "latest_events_invited_with_answers"
context = {'latest_events_invited':latest_events_invited, 'latest_answers_list':latest_answers_list}
else:
[...]
return render(request, 'events/index.html', context)
And the models.
Event
class Event(models.Model):
PRIVACY_CHOICES = (
(0, 'Public'),
(1, 'Invite only'),
)
name = models.CharField(max_length=200)
[...]
privacy = models.PositiveSmallIntegerField(max_length=1, choices=PRIVACY_CHOICES, default=0)
invited = models.ManyToManyField(settings.AUTH_USER_MODEL, through='Invite', related_name='events_invited', blank=True)
answers = models.ManyToManyField(settings.AUTH_USER_MODEL, through='Answer', related_name='events_answered', blank=True)
Invite
class Invite(models.Model):
user_invited = models.ForeignKey(settings.AUTH_USER_MODEL)
event = models.ForeignKey(Event)
created = models.DateTimeField(default=timezone.now, editable=False)
class Meta:
unique_together = (("user_invited", "event"),)
Answer
class Answer(models.Model):
ANSWER_CHOICES = (
(1, 'Attending'),
(2, 'Not attending'),
)
user_answering = models.ForeignKey(settings.AUTH_USER_MODEL)
event = models.ForeignKey(Event)
answer = models.PositiveSmallIntegerField(max_length=1, choices=ANSWER_CHOICES)
class Meta:
unique_together = (("user_answering", "event"),)
Hopefully someone here can help me out.
Thanks.

Answer 1

One option is to automatically add an Answer whenever someone is invited, with a default of No. Use the post_save signal of Invite or overwrite the save method so you don't have to create an associated Answer each time.
Another option, probably better, is to do the logic in the view rather than in the template. Check if an answer exists; if it does, pass a list of answers to the template; if not, pass an empty list.
EDIT: Or, better yet:
In the view:
try:
user_answer = Answer.objects.filter(user_answering = request.user).filter(event=event)
except ObjectDoesNotExist:
answer = None
EDIT2: Or how about:
Answer.objects.filter(user_answering = request.user).filter(event__in=Event.objects.filter(user__in = event.invited))

Answer 2

Ok, so I figured it out. I don't know if this is the optimal solution but it works for now.
In the end, it is only 2 queries, which I combine with for loops.
Here's the code: (views.py)
def index(request):
if request.user.is_authenticated():
latest_events_invited = Event.objects.filter(invite__user_invited=request.user)
latest_answers_for_events = Answer.objects.filter(user_answering=request.user, event__in=latest_events_invited)
for event in latest_events_invited:
for answer in latest_answers_for_events:
if answer.event.id == event.id:
event.answer = answer.answer
else:
event.answer = ''
context = {'latest_events_invited': latest_events_invited,}
return render(request, 'events/index.html', context)
I can then access the answer directly in the template (index.html) like so:
{{ event.answer }}