Templates and views (accessing field from other model)

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I'm trying to access the Theme name and put it in my template. It's been working fine until I set the Theme.name as primary key. Did the primary_key=True break it? How can I use it in template? I need the primary key to make the DetailView. Or am I completely wrong about everything?

#Template
{% for set in sets_list %}
     #...
     <b>Theme:</b> <a href="/themes/{{ set.theme }}/">{{ set.theme }}</a><br />
     <b>Subtheme:</b> <a href="/subthemes/{{ set.subtheme }}/">{{ set.subtheme }}</a>
     #...
{% endfor %}

#Models
class Set(models.Model):
    #...
    theme = models.ForeignKey('Theme')
    #...

class Theme(models.Model):
    name = models.CharField(max_length=32, primary_key=True, help_text="Primary key.")
    #...

#Views
urlpatterns = patterns('',
    (r'^sets/$', ListView.as_view(
        model=Set,
        context_object_name="sets_list",
        template_name='data/sets_list.html',
        )),
    (r'^sets/(?P<pk>\d+)/$', DetailView.as_view(
        model=Set,
        context_object_name="set_details",
        template_name='data/set_details.html',
        )),
)

urlpatterns += patterns('',
    (r'^themes/$', ListView.as_view(
        model=Theme,
        context_object_name="themes_list",
        template_name='data/themes_list.html',
        )),
    (r'^themes/(?P<pk>\d+)/$', DetailView.as_view(
        model=Theme,
        context_object_name="theme_details",
        template_name='data/theme_details.html',
        )),
)