Django: Organize objects in a dictionary by foreign key

Question

I have this problem: I've got two models, products and categories, the categories are defined in a fixture (they won't change), and I need to display all categories in a single template, as a grid. The models are rather simple, the important thing is that Products have a foreign key pointing to Category model, and another one pointing to User model (owner).

I have to display all products in each category block (CSS doesn't matter, I just need to be able to bring them up) but the solution I've got so far is this

View

def index(request):
    user_products = []
    if request.user.is_authenticated():
        user_products = Product.objects.filter(owner=request.user)
    categories = Category.objects.all()
    return render(request, 'index.html', {'user_products': user_products, 'categories': categories})

Template

<!-- This goes for each category, 12 in total -->
<div>
    <h3>Category (name hardcoded)</h3>
    {% for category in categories %}
        {% if category.pk == 3 %}
            <ul class="product_list">
            {% for product in category.product_set.all %}
                {% if product.owner == request.user %}
                <li>
                    <div class="product">
                        <h2 class="title">{{ product.title }} </h2>
                        <p class="text">{{ product.text }}</p>
                    </div>
                </li>
                {% endif %}
            {% empty %}
            {% endfor %}
            </ul>
        {% endif %}
    {% endfor %}
</div>

I want send in context something like:

user_products = {<category.pk>: [list of Product objects in category]}

so I can access the product list of each category without defining a for loop each time and filtering them by request.user . Can you think of a better way to do this? I had to hardcode each category because they have a given order, but if you know how to display them dynamically while keeping that order it would be great. Thanks!

Answer 1

There is better way to do this. Use Prefetch object. Prefetch will have only filtered data for each category.

from django.db.models.query import Prefetch

def index(request):
    user_products = []
    if request.user.is_authenticated():
        user_products = Product.objects.filter(owner=request.user)
    else:
        # answering comment
        user_products = Product.objects.none()
    prefetch = Prefetch('product_set', queryset=user_products)
    categories = Category.objects.all().prefetch_related(prefetch)

    return render(request, 'index.html', {'categories': categories})

And then you can do this in template

<!-- You need to do this only once. No need to copy this for each category -->
<div>
    {% for category in categories %}
    <h3>Category {{ category.name }}</h3>
        <ul class="product_list">
        {% for product in category.product_set.all %}
            <li>
                <div class="product">
                    <h2 class="title">{{ product.title }} </h2>
                    <p class="text">{{ product.text }}</p>
                </div>
            </li>
        {% endfor %}
        </ul>
    {% endfor %}
</div>