Command to list models related to a given model grouped by the on_delete behavior. Useful if you want to figure out what will happen if you delete an instance of a model.

relations.py

from django.core.management.base import BaseCommand
from django.apps import apps
from django.db.models import ForeignKey, OneToOneField


class Command(BaseCommand):
    """
    Command to list models related to a given model grouped by the on_delete behavior.

    It shows which models have relationships with the target model and groups them
    by the type of on_delete action (CASCADE, SET_NULL, DO_NOTHING, etc.) along with
    a description of what that action means.

    Usage:
        python manage.py relations app_label.ModelName
    """

    ON_DELETE_DESCRIPTIONS = {
        'CASCADE': 'Deletes related instances when the primary instance is deleted.',
        'SET_NULL': 'Sets the related field to NULL when the primary instance is deleted (field must be nullable).',
        'DO_NOTHING': 'Does nothing automatically; manual handling required and may cause errors if database constraints are violated.',
        'PROTECT': 'Prevents deletion if related objects exist, raises integrity error.',
        'SET_DEFAULT': 'Sets the related field to its default value when the primary instance is deleted.',
        'RESTRICT': 'Prevents deletion if related objects exist (similar to PROTECT).',
    }

    def add_arguments(self, parser):
        parser.add_argument(
            'model_name',
            type=str,
            help='Model name in the format app_label.ModelName'
        )

    def handle(self, *args, **options):
        model_name = options['model_name']
        try:
            app_label, model_class_name = model_name.split('.')
            model = apps.get_model(app_label, model_class_name)
        except (ValueError, LookupError):
            self.stdout.write(self.style.ERROR(f'Model not found or incorrect format: {model_name}'))
            return

        grouped_models = {}

        # Iterate all registered models to find fields pointing to the target model
        for m in apps.get_models():
            for field in m._meta.get_fields():
                if isinstance(field, (ForeignKey, OneToOneField)):
                    # field.remote_field contains metadata about how the relationship is configured, including the
                    # target model, what to do on deletion
                    if field.remote_field and field.remote_field.model == model:
                        on_delete_action = field.remote_field.on_delete.__name__
                        grouped_models.setdefault(on_delete_action, []).append(m._meta.label)

        self.stdout.write(f'Relations for {model_name} grouped by on_delete type:\n')

        if not grouped_models:
            self.stdout.write('- (No specific on_delete relations found)')
            return

        # Print each on_delete group with description and related models
        for action, models_list in sorted(grouped_models.items()):
            description = self.ON_DELETE_DESCRIPTIONS.get(action, 'No description available.')
            self.stdout.write(f'{action}:')
            self.stdout.write(f'  {description}')
            for model_label in sorted(set(models_list)):
                self.stdout.write(f'- {model_label}')
            self.stdout.write('')

        self.stdout.write(self.style.SUCCESS('Analysis completed.'))