gwift-book/source/part-4-services-oriented-applications/trees.adoc

=== Arborescences

[source,python]
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[source,python]
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# <app>/management/commands/rebuild.py

"""This command manages Closure Tables implementation

It adds new levels and cleans links between entities.
This way, it's relatively easy to fetch an entire tree with just one tiny request.

"""

from django.core.management.base import BaseCommand

from rps.structure.models import Entity, EntityTreePath


class Command(BaseCommand):
    def handle(self, *args, **options):
        entities = Entity.objects.all()

        for entity in entities:
            breadcrumb = [node for node in entity.breadcrumb()]

            tree = set(EntityTreePath.objects.filter(descendant=entity))

            for idx, node in enumerate(breadcrumb):
                tree_path, _ = EntityTreePath.objects.get_or_create(
                    ancestor=node, descendant=entity, weight=idx + 1
                )

                if tree_path in tree:
                    tree.remove(tree_path)

            for tree_path in tree:
                tree_path.delete()
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