CODEWITHSUNDEEP
djangodjango-models
Alex
Alex

Reputation: 6047

related items in loop creates many duplicate queries

Rendering a page gets really slow when related objects are requested in a template. 

class Item(models.Model):
   name =  models.CharField(max_length=160)
   ...

class Box(models.Model):
   ...
   items = models.ForeignKey(Item, on_delete=models.CASCADE, null=True) 

#template  
{% for item in items %}  
   {{ item.box_set.first }}  
   {{ item.box_set.latest }}  
{% endfor %}

Debug toolbar shows there many duplicate queries. Why is this happening? Is there a way to speed this up?

Upvotes: 0

Views: 26

Answers (1)

schillingt
schillingt

Reputation: 13731

The Django ORM has to make a request to the database when accessing a related field unless it's already cached. The main ways of caching the related objects are via select_related and prefetch_related.

What you're trying to do is a bit more difficult; you're trying to get two specific items from a collection. You can use .annotate() and Subquery to pull singular fields from a related model. This would be useful if you just wanted to display single field from box, but if you need the whole box instance this won't work.

Upvotes: 1

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