Django Queryset: group by two field values

Question

I have a model like this

Class ExchangeMarket(models.Model):
base_currency = models.ForeignKey(Currency)
quote_currency = models.ForeignKey(Currency)
... various other fields

And some entries like

base: EUR, quote: USD ...
base: EUR, quote: USD ...
base: USD, quote: EUR ...
base: BTC, quote: USD ...
...

I need to find entries sharing the base/quote combination, i.e. in my example that's the first three

How to create such a queryset? If I do something like

ExchangeMarket.objects.all().values('base_currency', 'quote_currency').annotate(pks=ArrayAgg('pk'))

I will only find exact matches with same bases and quotes (like EUR/USD, first two of my entries) while I need to find both EUR/USD and USD/EUR. Thanks!

Answer 1

You can work with Least ^[Django-doc] and Greatest ^[Django-doc] here:

from django.db.models.functions import Greatest, Least

ExchangeMarket.objects.values(
    cur1=Least('base_currency', 'quote_currency'),
    cur2=Greatest('base_currency', 'quote_currency'),
).annotate(pks=ArrayAgg('pk'))

This will then add the primary key to the two same items.

That being said, I would advise to do some post-processing in Python, and work with .groupby(…) ^[Python-doc]:

from itertools import attrgetter

from django.db.models.functions import Greatest, Least

items = {
    k: list(vs)
    for k, vs in groupby(
        ExchangeMarket.objects.annotate(
            cur1=Least('base_currency', 'quote_currency'),
            cur2=Greatest('base_currency', 'quote_currency'),
        ).order_by('cur1', 'cur2'),
        attrgetter('cur1', cur2),
    )
}

We even can do additional processing where we also add the (k2, k1) entry to the dictionary, if the (k1, k2) entry is already passed:

for k1, k2 in list(items):
    items[k2, k1] = items[k1, k2]

This means we can obtain the ExchangeMarket objects for items['EUR', 'USD'] and items['USD', 'EUR'].