distinct_pairs.py

import datetime
from typing import Dict, Any

from hypothesis import strategies
from django.db import models
from django.db.models import F, When, Case


class MessageManager(models.Manager):
    def latest_distinct(self):
        return super().get_queryset().annotate(
            user_1=Case(
                When(sender__gt=F('recipient'), then=F('recipient')),
                default=F('sender')
            ),
            user_2=Case(
                When(sender__gt=F('recipient'), then=F('sender')),
                default=F('recipient')
            )
        ).order_by('user_1', 'user_2', '-when').distinct('user_1', 'user_2')


class Message(models.Model):
    when = models.DateTimeField(auto_now=True, db_index=True)
    sender = models.CharField(max_length=127)
    recipient = models.CharField(max_length=127)

    objects = MessageManager()

    def __str__(self):
        return f'[{self.when.ctime()}] {self.sender} -> {self.recipient}'


def example_message() -> Dict[str, Any]:
    users = [
        'batman', 'superman', 'wonder-woman', 'flash', 'green-lantern', 'martian-manhunter',
        'joker', 'darkseid', 'lex-luthor', 'ananta-jalil', 'zoom', 'savitr', 'mr-mxyzpltk',
        'victor-zsasz', 'hawkgirl', 'guardian', 'super-girl',
    ]
    sender = strategies.sampled_from(users).example()
    recipient = strategies.sampled_from([u for u in users if u != sender]).example()
    when = strategies.dates(min_value=datetime.date(2019, 1, 1),
                            max_value=datetime.date.today()).example()

    return {
        'sender': sender,
        'recipient': recipient,
        'when': when
    }


def seed(limit: int = 1000) -> None:
    for _ in range(limit):
        Message.objects.create(**example_message())