Question

This is a bit of a puzzle, I have these pseudo models:

class Country(models.Model):
   name = models.CharField(unique=True)

class Region(models.Model):
   name = models.CharField(unique=True)
   country = models.ForeignKey(Country)

class SubRegion(models.Model):
   name = models.CharField(unique=True)
   region = models.ForeignKey(Region)

class Estate(models.Model):
   name = models.CharField(unique=True)
   sub_region = models.ForeignKey(SubRegion)

I am trying to JSON serialize their data as below. However i’m not sure how to do this effectively (avoiding too much database queries), suggestions are appreciated

{

    CountryX: { 
        RegionX {
            SubRegionX = [
                "EstateX"
                "EstateY",
                "EstateZ"
            ],
            SubRegionY = [ etc... ]
        },
        RegionY { etc... }
    },
    CountryY: { etc... }

}

Answer 1

I haven’t tested this, but it should give you the idea. Start with the innermost object, use select_related to traverse the heirarchy, then loop over the innermost objects, adding the keys for the heirarchy as needed.

Just a note of warning, if there are Countries/Regions/Subregions without any estates, they won’t be included in the JSON. If that’s not OK, you’ll need to query each of the models separately.

data = {}
for e in Estate.objects.select_related("sub_region__region__country"):
    sub, region, country = e.sub_region, e.sub_region.region, e.sub_region.region.country
    if country.name not in data:
        data[country.name] = {}
    if region.name not in data[country.name]:
        data[country.name][region.name] = {}
    if sub.name not in data[country.name][region.name]:
       data[country.name][region.name][sub.name] = []
    data[country.name][region.name][sub.name].append(e.name)
json_data = json.dumps(data)