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POReverse relation (ForeignKey) in Djano ORM with unique combination
 

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  1. POReverse relation (ForeignKey) in Djano ORM with unique combination
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    <p>Hi I was just wondering if this is feasible through Django ORM:</p> <p>Please ignore model names etc, they are not real</p> <pre><code>class Image(models.Model): ... item = models.ForeignKey(Bag, related_name="images") flag_frontpage_image = models.BooleanField() imagefile = models.ImageField(upload_to="items/%m/%Y") ... class Item(models.Model): ... </code></pre> <p>In views.py:</p> <pre><code>class LandingPageView(SomeMixin,SomeOtherMixin,ListView): .... def get_context_data(self, **kwargs): context = super(LandingPageView, self).get_context_data(**kwargs) context['items'] = Item.objects.prefetch_related('images').order_by('?').all()[:10] return context </code></pre> <p>This is ok and working ok, what I want to achieve is that I want in the view to fetch in 1 query applying a join the image a well. LandingPageView displays a list of items with the image that is flagged as frontpage image, as Django states (and SQL wise correctly) a reverse select_related is not possible, since Item can have many images, but...a Item can have only one frontpage image. I know there are other ways of achieving this in terms of refactoring, currently since I prefer to avoid partitioning models, I follow a mixin way:</p> <pre><code>class GetFeaturedMixin(object): @property def featured_image(self): if self.images.count &gt; 0: for image in self.images.all(): if image.flag_frontpage_image == 1: return image.imagefile class Item(models.Model, GetFeaturedMixin): ... </code></pre> <p>And then in the view:</p> <pre><code>&lt;img src="{{ item.featured_image }}"&gt; </code></pre> <p>This works fine, but I would like to know if there is a way through Django relational querysets to achieve fetching the whole set in single queryset. Please don't answer or ask about raw queries, I know they do exist and have used them even on this type, I was just wondering if the above can be achieved with the ORM layer. </p>
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    1. USpetkostas
    1. USpetkostas
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    1. CO`self.images.filter(flag_frontpage_image=True)` is obviously better than pythonically iterating over all of them.
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      1. USyuvi
    2. COBut that creates additional queries, since this is a reverse lookup, that's why I iterate through them with the above method, since the initial prefetch_related fetches all images for the items. I am gonna check again to make sure though
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      1. POReverse relation (ForeignKey) in Djano ORM with unique combination
      1. USpetkostas
    3. COChecked, I get 1 additional query for every call for featured_image so not preferred compared to the iteration.
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      1. USpetkostas
 

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