An example of the N+1 database insertion problem in django

n_plus_one_in_django.py

from django.db.models import CharField, Model, OneToOneField, AutoField, EmailField


class User(Model):
    id = AutoField(primary_key=True)  # The default Id field from django
    username = CharField(max_length=30)
    profile = OneToOneField(Profile, primary_key=True, related_name='user')


class Profile(Model):
    id = AutoField(primary_key=True)  # The default Id field from django
    primary_email = EmailField()
    secondary_email = EmailField()


def bulk_insert(list_of_user_details):
    """ Demonstrates the N+1 problem using a bulk creation of users example

    Questions to think about:
        - Why does the first code block create a ValueError?
        - We can use bulk_create on the User model, but not the Profile model or both at once. Why not?
        - What happens when instead of 2 users, the bulk_insert is done for 10 users? 20,000 users? 10m users?
        - What happens when there's an error halfway through the insertion process?

    """

    # First of all, why does the following produce a ValueError like:
    #     Traceback (most recent call last):
    #     ...
    #     ValueError: save() prohibited to prevent data loss due to unsaved related object 'profile' ?
    try:
        # Why does this produce a value error?
        user_details = list_of_user_details[0]
        user = User(username=user_details['username'])
        profile = Profile(primary_email=user_details['primary_email'], secondary_email=user_details['secondary_email'])
        user.profile = profile
        user.save()

    except ValueError:
        pass

    # Django has bulk creation for models... Let's just add them all at once! But why won't the following work?
    try:
        users = []
        for user_details in list_of_user_details:
            user = User(username=user_details['username'])
            user.profile = Profile(primary_email=user_details['primary_email'], secondary_email=user_details['secondary_email'])
            users.append(user)
        User.objects.bulk_create(users)

    except ValueError:
        pass

    # The following works! But what's the problem with it?
    try:
        users = []
        for user_details in list_of_user_details:
            user = User(username=user_details['username'])
            profile = Profile(primary_email=user_details['primary_email'], secondary_email=user_details['secondary_email'])
            profile.save()
            user.profile = profile
            users.append(user)
        User.objects.bulk_create(users)

    except ValueError:
        pass

    # How can we get over the problem in the previous code block?


bulk_insert([{'username': 'joebloggs',   'primary_email': '[email protected]',   'secondary_email': '[email protected]'},
             {'username': 'anniebloggs', 'primary_email': '[email protected]', 'secondary_email': '[email protected]'}])