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PODjango -- Common Scenario: Product with M2M -> Categories. How do I efficiently query all categories on a product queryset?
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copied!<p>I'm trying to cut down on my query counts and have a question.</p> <p>I have a typical product model set up that can have multiple categories.</p> <p><code>Product M2M -> Category</code></p> <p>I have a function defined that pulls the main category (lets just say its the latest one): </p> <pre><code>class Category(models.Model): name = models.CharField(max_length=1) class Product(models.Model): category = models.ManyToManyField(Category) def main_category(self): return self.category.latest('id') # or any other way to determine main category </code></pre> <p>I iterate through products in my template, displaying its main category.</p> <pre><code>{% for product in products %} Name: {{ product.name }} Category: {{ product.main_category }} {% endfor %} </code></pre> <p>This causes a query for every product. How can I get it all in less queries, in Python?</p> <p>For my other <code>MultipleObjectFK -> Product</code> models I've been able to <a href="http://www.slideshare.net/zeeg/db-tips-tricks-django-meetup" rel="nofollow">partition my queries (slideshow)</a> and use just 2 queries + python, but I can't seem to apply it to M2M because when I pull the Categories that are referenced in my product queryset, I have no idea which product triggered the category match.</p> <p>I'm leaning towards setting up a <code>main_category</code> field on my model that has a category ID calculated whenever there's an M2MField changed signal.</p> <p>Thanks for your time : )</p>

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