django prefetch_related on multiple models

Question

I have models like this

Model Trip:
  hotel: m2m_field to Hotel
  flight: m2m_field to Flight
Model Hotel:
  city: foreignkey to City
Model Flight:
  city: foreignkey to City

The problem is when I do

Trip.objects.prefetch_related('hotel__city', 'flight__city')

It generates 5 sql queries: 1 on Trip, 1 on Hotel, 1 on Flight, 1 on City from Hotel result, and 1 on City from Flight result. Can I reduce the last two queries on City table into 1? I think I may be asking too much from ORM, but just want to know whether it's possible either in ORM or raw-query.

squarelogic.hayden · Accepted Answer · 2015-08-14 14:35:15Z

1

prefetch_related will always perform separate queries - from the docs:

prefetch_related(**lookups) - Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups.

If you're looking to combine queries, you should be using select_related, which in this instance will perform a single query with four joins to the hotel, flight & city(x2) tables.

answered Aug 14, 2015 at 14:35

squarelogic.hayden

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