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)