Django Queryset order by a not directly related model's field

Question

I have the following model design:

class Boat(models.Model):    
    name = models.CharField(max_length=30, null=False)
    harbour = models.ForeignKey(Harbour, null=True, on_delete=models.SET_NULL)

class Harbour(models.Model):
    name = models.CharField(max_length=60)
    city = models.ForeignKey(City, null=True, related_name='city_harbours',  on_delete=models.SET_NULL)

class City(models.Model):
    name = models.CharField(max_length=80)
    county = models.CharField(max_length=30, null=True)


class NearestCity(models.Model):
    city = models.ForeignKey(City, related_name='city', null=False, on_delete=models.CASCADE)
    near_city = models.ForeignKey(City, related_name='near_city', null=False, on_delete=models.CASCADE)
    weight = models.DecimalField(null=False, max_digits=25, decimal_places=20)

Brief explanation:

• A Boat belongs to a Harbour and a Harbour belongs to a City .

• Not all Cities have a harbour related.

• NearestCity is a table where is stored how close is a city the rest of the cities: The weight is a decimal value that indicates how far is city from _near_city_. The more smaller is the 'weight' value, the more close is city to near_city. For example:

city     near_city      weight
----    ---------       ------
London   Rome           2.210103
London   Manchester     0.113134

It means that Manchester is closer to London than Rome.

Problem to solve:

Given a name of a city without any harbour related, for instance Berlin, a want to return all the boats of those closest cities that do have at least one harbour related. This Boat's queryset must be ordered by weight DESC.

I am really newbie with django queryset and I tried to solved using annotate with subqueries , aggregations , etc.. but I could not achieve it.

Answer 1

I would try raw sql . Something like this should work:

city = selected_city_pk
sql = """
    SELECT b.name, nc.weight, nc.near_city_id
    FROM appname_nearestcity nc
    JOIN appname_city c ON near_city_id = c.id
    JOIN appname_harbour h ON near_city_id = h.city_id
    JOIN appname_boat b ON h.id = b.harbour_id
    WHERE nc.city_id = %s
    ORDER BY nc.weight, b.name
"""
results = Boat.objects.raw(sql, [city,])

for r in results: print(r.id, r.name, r.weight, r.near_city_id)
# boat_id, boat_name, weight, near_city_id
>> 14 b-0-h-sochi-0 10 6
>> 15 b-0-h-sochi-1 10 6
>> 16 b-0-h-sochi-2 10 6
>> 17 b-1-h-sochi-2 10 6
>> 18 b-2-h-sochi-2 10 6
>> 11 b-0-h-rome-0 55 5
>> 12 b-1-h-rome-0 55 5
>> 13 b-2-h-rome-0 55 5
>> 4 b-0-h-brasilia-0 56 4
>> 7 b-0-h-brasilia-1 56 4
>> 10 b-0-h-brasilia-2 56 4
>> 5 b-1-h-brasilia-0 56 4
>> 8 b-1-h-brasilia-1 56 4
>> 6 b-2-h-brasilia-0 56 4
>> 9 b-2-h-brasilia-1 56 4
>> 1 b-0-h-beijin-0 93 2
>> 3 b-0-h-beijin-1 93 2
>> 2 b-1-h-beijin-0 93 2

Remember to replace table names with actual table names from your DB because Django adds app name prefixes to them.