在Django模板中，如何确定request.user是否在与该用户关联的对象列表中？

Question

I've got a Django model that looks like this

from django.contrib.auth.models import User
class Cohort(models.Model) :
    cohort_name = models.CharField(max_length=64, primary_key=True)
    cohort_description = models.TextField(null=False)
    creation_date = models.DateTimeField(default=now)

class CohortMembers(models.Model) :
    cohort = models.ForeignKey(Cohort)
    member = models.ForeignKey(User)
    creation_date = models.DateTimeField(default=now)

So you can see there is a many-to-many relationship between Cohorts and Users.

In a template, I am listing Cohorts as such (simplified to give you the idea):

{% for cohort in object_list %}
  <a href="./{{ cohort.cohort_name }}/">{{ cohort.cohort_name }}</a>
  <!-- list all members of the cohort -->
  {% for cohortmember in cohort.cohortmembers_set.all %}
    {% if request.user.username == cohortmember.member.username %}
      <!-- the user is a member of the cohort, provide a delete button -->
      <button>Leave cohort</button>
    {% else %}
      Some other user called {{ cohortmember.member.username }} is a member.
    {% endif %}
  {% endfor %}
{% endfor %}

What I need to do, is provide a button for the user to join the cohort if they are not already a member. In the most primitive of python, you might do this;

# clearly some better python would be to use a django model query, rather
# than iterate over the members, but this encapsulates the basic logic
is_member = False
for cohortmember in cohort.cohortmembers_set.all:
     if request.user == cohortmember.member:
        is_member = True
if is_member :
     # a leave button
else:
     # a join button

Can I do any of that in the Django template language or do I have to resort to putting code like that into the View class?

Answer 1

The only real way I have found to do this is a combination of manipulating the CohortView and a simple bit of template programming.

I turned the question around: not to ask "is this user a member of that cohort?" but made it into "is this cohort in the list of the user's memberships?" in this simple way.

In the CohortListView's get_context_data as follows:

def get_context_data(self, **kwargs) :
    context = super(CohortListView, self).get_context_data(**kwargs)
    memberships = CohortMembers.objects.filter(member=self.request.user)
    cohorts = []
    for membership in memberships:
        cohorts.append(membership.cohort)
    context['memberships'] = cohorts
    return context

Note I'm abandoning the many to many object and just storing a list of Cohorts the user is a member of directly in the context.

Then, in the template:

{% for cohort in object_list %}
  {% if cohort in memberships %}
    <!-- leave button -->
  {% else %}
    <!-- join button -->
  {% endif %}
{% endfor %}