Django ORM cookbook

Django ORM cookbook is a set of recipes of how to do things with Django. They take the form of about 50 questions of the form How to do X with Django ORM/Queryset.

We have a set of models which we use across the book for answering these questions.

The models

You plan to write a set of models and an assoicated admin for UMSRA researchers. You come up with two apps entities and events. The models are

Events

from django.db import models
from entities.models import Hero, Villain

class Epic(models.Model):
    name = models.CharField(max_length=255)
    participating_heroes = models.ManyToManyField(Hero)
    participating_villains = models.ManyToManyField(Villain)


class Event(models.Model):
    epic = models.ForeignKey(Epic, on_delete=models.CASCADE)
    details = models.TextField()
    years_ago = models.PositiveIntegerField()


class EventHero(models.Model):
    event = models.ForeignKey(Event, on_delete=models.CASCADE)
    hero = models.ForeignKey(Hero, on_delete=models.CASCADE)
    is_primary = models.BooleanField()


class EventVillain(models.Model):
    event = models.ForeignKey(Event, on_delete=models.CASCADE)
    hero = models.ForeignKey(Villain, on_delete=models.CASCADE)
    is_primary = models.BooleanField()

Entities

from django.db import models


class Category(models.Model):
    name = models.CharField(max_length=100)


class Origin(models.Model):
    name = models.CharField(max_length=100)


class Entity(models.Model):
    GENDER_MALE = "Male"
    GENDER_FEMALE = "Female"
    GENDER_OTHERS = "Others/Unknown"

    name = models.CharField(max_length=100)
    alternative_name = models.CharField(
        max_length=100, null=True, blank=True
    )


    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    origin = models.ForeignKey(Origin, on_delete=models.CASCADE)
    gender = models.CharField(
        max_length=100,
        choices=(
            (GENDER_MALE, GENDER_MALE),
            (GENDER_FEMALE, GENDER_FEMALE),
            (GENDER_OTHERS, GENDER_OTHERS),
        )
    )
    description = models.TextField()

    class Meta:
        abstract = True


class Hero(Entity):

    is_immortal = models.BooleanField(default=True)

    benevolence_factor = models.PositiveSmallIntegerField(
        help_text="How benevolent this hero is?"
    )
    arbitrariness_factor = models.PositiveSmallIntegerField(
        help_text="How arbitrary this hero is?"
    )
    # relationships
    father = models.ForeignKey(
        "self", related_name="+", null=True, blank=True, on_delete=models.SET_NULL
    )
    mother = models.ForeignKey(
        "self", related_name="+", null=True, blank=True, on_delete=models.SET_NULL
    )
    spouse = models.ForeignKey(
        "self", related_name="+", null=True, blank=True, on_delete=models.SET_NULL
    )


class Villain(Entity):
    is_immortal = models.BooleanField(default=False)

    malevolence_factor = models.PositiveSmallIntegerField(
        help_text="How malevolent this villain is?"
    )
    power_factor = models.PositiveSmallIntegerField(
        help_text="How powerful this villain is?"
    )
    is_unique = models.BooleanField(default=True)
    count = models.PositiveSmallIntegerField(default=1)