m2m through : accessing intermediate table from templates - django

Question

When a ManyToMany relationship has extra data via a through table, how can you get to the data in a template? From a view I can get the data if I supply parameters:
class Category(models.Model):
title = models.CharField(max_length=1024,null=True,blank=True)
entry = models.ManyToManyField(Entry,null=True,blank=True,
related_name='category_entry',
through='CategoryEntry',
)
class CategoryEntry(models.Model):
category = models.ForeignKey(Category)
entry = models.ForeignKey(Entry)
votes = models.IntegerField(null=False, default=0)
def category_detail(request, pk):
category = models.Category.objects.select_related().get(pk=pk)
entries = category.entry.order_by('-temp_sort_order').filter(temp_sort_order__gte=0)
for entry in entries:
assert isinstance(entry, models.Entry)
ce = models.CategoryEntry.objects.get(entry=entry, category=category)
pprint('Show votes as a test: ' + ce.votes) #OK
pprint('entry title: ' + entry.title) #OK
pprint('entry votes: ' + str(entry.category_entry.votes)) #BAD
pprint('entry votes: ' + str(entry.entry.votes)) #BAD
....
But templates can't supply parameters to methods.
The documentation at https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships is silent on templates. Using using for entry in category.category_entry_set.all gives 'Category' object has no attribute 'category_entry_set'. category.category_entry.all does not work either.
Ultimately I want to display the extra data in a template:
{% for entry in entries %}
<ul>
<li>Title: {{ entry.title }} Votes: {{ entry.category_entry.votes }} {{ entry.entry.votes }}</li>
</ul>
{% endfor %}

Answer 1

If you have a category instance in template:
category.entry.all -> list of entries
If you have an entry instance in template:
entry.category_entry.all -> list of categories
You should call M2M fields in plural form,
then you will have a more readable code
category.entries.all
%model%_set syntax (or related name, if you've specified it) is using to access to model trough a backward relationship.
https://docs.djangoproject.com/en/1.4/topics/db/queries/#following-relationships-backward
But how do I get the 'votes' associated with the m2m instance? – Bryce
I suggest you the following way:
class Category(models.Model):
title = models.CharField(max_length=1024,null=True,blank=True)
entries = models.ManyToManyField(Entry,null=True,blank=True,
related_name='categories',
through='CategoryEntry',
)
class CategoryEntry(models.Model):
category = models.ForeignKey(Category, related_name='category_entries')
entry = models.ForeignKey(Entry)
votes = models.IntegerField(null=False, default=0)
def category_detail(request, pk):
category = models.Category.objects.select_related().get(pk=pk)
category_entries = category.category_entries.filter(entry__temp_sort_order__gte=0).order_by('-entry__temp_sort_order')
for category_entry in category_entries:
# category_entry is an instance of the model CategoryEntry
pprint('category entry votes: ' + str(category_entry.votes))
pprint('entry title: ' + category_entry.entry.title)
....
HOW TO
entry = Entry.objects.get(pk=1)
entry.categories.all() # list of categories (here we work through related name of the field entries)
category = Category.objects.get(pk=1)
category.entries.all() # list of entries (here we work through m2m field entries)
category.category_entries.all() # list of CategoryEntry objects (through related name category_entries of the field category in model CategoryEntry)

Answer 2

Updating my answer, i mistakenly put related manager on wrong model, in your case, like Andrey said, the correct way to get entries from category is:
category.entry.all()
Now, to address your iteration and ordering question. In python it will look like this:
for ce in category.categoryentry_set.order_by('-votes'):
print ce.entry, ce.votes
This will give you entries in each category ordered by votes. To get this to template you can just save a queryset category.categoryentry_set.order_by('-votes') into variable and iterate over it.

Answer 3

Here's an ugly ugly hack that works. After the filter and sort, process the list and append the extra model fields. The templates now have easy access:
entries = category.entry.order_by('-temp_sort_order').filter(temp_sort_order__gte=0)
for entry in entries:
assert isinstance(entry, models.Entry)
ce = models.CategoryEntry.objects.get(entry=entry, category=category)
entry.xxx_votes = mark_safe(ce.votes) # use {{ entry.xxx_votes to access }}
entry.xxx_ce = ce # Use {{ entry.ce.votes to access }}
return render_to_response('category.html')
Hopefully someone can provide a better answer, or suggest an improvement to django itself. This solution does not allow me to sort: category.entry.order_by('-category_entry.votes')