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I'm unsure of the best way to design my models for this task. Searching around suggests that I need to use something like Django-MPTT.

I am creating a listings application where any 'item' can be listed under multiple categories. From any category I will need to be able to query for 'items' that match.

An example should illustrate what I want a little better.

I have one Item1 that is to be assigned to two sub categories.

TopLevel1
   --- Sublevel1
   --- Sublevel2
           -----> Item1

TopLevel1
   --- Sublevel1
          ------> Item1
   --- Sublevel2

Can anyone suggest how the models/relationships should be constructed? My current implementation 

class Category(models.Model):
    name = models.CharField(max_length=128, blank=False)
    parent = models.ForeignKey('self', null=True, blank=True)


class Item(TimeStampedModel):
    name = models.TextField(null=False, blank=False)
    categories = models.ManyToManyField(Category)

Does not feel like the correct solution?

J T
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1 Answers1

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This is how I see it...

Your models seem close, a m2m seems appropriate, but your diagram would look more like:

Item1
    ---> TopLevel1 (accessed through `categories`)
        ---> SubLevel1 (accessed through `Category.children`, see below)
        ---> SubLevel2 (accessed through `Category.children`, see below)

and your models would look more like 

class Category(models.Model):
    name = models.CharField(max_length=128) # `blank` is false by default
    parent = models.ForeignKey('self', null=True, blank=True, related_name='children') # by giving this field a nice `related_name`, we can easily access the linked categories

class Item(TimeStampedModel):
    name = models.TextField() # `null` and `blank` are false by default
    categories = models.ManyToManyField(Category, null=True, blank=True, related_name='items')

Now, if you have an item and would like to get it's top-level categories, you can loop through item.categories.all()

{% for category in item.categories.all %}
    Top Level: {{ category.name }}
{% endfor %}

and to access the sub-level categories, you loop through children.all() in each category

{% for category in item.categories.all %}
    Top Level: {{ category.name }}
    {% for child in category.children.all %}
        Sub Level: {{ child.name }}
        Parent: {{ child.parent.name }}
    {% endfor %}
{% endfor %}

you can also get all of the items in a category

{% for item in category.items.all %}
    Item: {{ item.name }}
{% endfor %}

or if you are in a top-level category

{% for child in category.children.all %}
    {{ child.name }}
    {% for item in child.items.all %}
        Item: {{ item.name }}
    {% endfor %}
{% endfor %}

And with

category_one = Category.objects.create('category one')
category_two = Category.objects.create('category two')
item_one = Item.objects.create('item one')

You can add a Category through the related manager categories on Item 

item_one.categories.add(category_one, category_two)

or you can add an Item through the related manager items on Category

category_one.items.add(item_one)
Ngenator
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  • Excellent thanks. I will read this in more detail and try it later. It seems exactly what I want. Thanks for the detailed response. – J T May 13 '13 at 11:10
  • This seems to be exactly what I want. I am experimenting with this in some tests. For efficiency, should the Category Model subclass MPTTModel and the ForeignKeyField be a TreeForeignKey? – J T May 13 '13 at 21:28
  • No problem. I'm unblocked enough now and will leave the mptt work until its worth looking at optimisations. I can probably just use a cache to avoid performance issues in this instance anyway. – J T May 14 '13 at 20:42