Add MP_Node to existing model

[thiras]

I'm trying change one of my models to MP_Node. When I try to makemigrations, it asks one-time depth and path values. I've couldn't find those default values in the documentation. What should be the default values of those?

Also it might good idea to add instructions about that to documentation.

[jrief]

NS_Node and MP_Node have completely different structures. If you want to migrate one into another, create a second model inheriting from MP_Node, then traverse the source tree, and for each node create a corresponding copy in your destination tree. After that remove the source tree, and rename the destination to the old name.

[helderco]

The OP didn't specifically say the existing model is an NS_Node. In my case, I'm trying to add hierarchy to a normal django model.

I can understand right away that depth should be 2, and I should create a RunPython migration operation to add the root and move all others in as children.

But what about path? And is there any command that has to be run afterward to insure integrity?

[helderco]

Here's how I did my migration. I wanted all existing nodes to be root nodes.

from django.db import migrations, models
from treebeard.mp_tree import MP_Node

# update if needed
steplen = MP_Node.steplen
alphabet = MP_Node.alphabet
node_order_by = ["name"]


def update_paths(apps, schema_editor):
    Node = apps.get_model("nodes", "Node")
    nodes = Node.objects.all().order_by(*node_order_by)
    for i, node in enumerate(nodes, 1):
        # for default values, this resolves to: "{:04d}".format(i)
        node.path = f"{{:{alphabet[0]}{steplen}d}}".format(i)
    if nodes:
        Node.objects.bulk_update(nodes, ["path"])


def backwards(apps, schema_editor):
    """nothing to do"""


class Migration(migrations.Migration):
    dependencies = [
        ("nodes", "0001_initial"),
    ]
    operations = [
        migrations.AddField(
            model_name="node",
            name="depth",
            field=models.PositiveIntegerField(default=1),
            preserve_default=False,
        ),
        migrations.AddField(
            model_name="node",
            name="numchild",
            field=models.PositiveIntegerField(default=0),
        ),
        migrations.AddField(
            model_name="node",
            name="path",
            field=models.CharField(default="", max_length=255),
        ),
        migrations.RunPython(update_paths, backwards),
        migrations.AlterField(
            model_name="node",
            name="path",
            field=models.CharField(max_length=255, unique=True),
        ),
    ]

Notice how I remove the unique validation when adding the path field because of existing rows. After updating paths, it's added again.

As for numchild, it already has the right default and for depth I give a value of 1 (root), only in the migration (preserve_default=False).

[buczek]

node.path = f"{{:{alphabet[0]}{steplen}d}}".format(i)

Doesn't use the alphabet at all, just the first character? '{:04d}'.format(i) produces legal paths , but only if alphabet happens to start with "0".

Alternative (coded here for a "Group" model, not for a "Node" model):

from treebeard.mp_tree import MP_Node

def update_path(apps, schema_editor):

    class Node(MP_Node):
        # may overwrite, should match new model:
        # steplen = 4
        # alphabet = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
        # node_order_by = []
        pass;

    Group = apps.get_model("personal","Group")
    db_alias = schema_editor.connection.alias

    for i, group in enumerate(Group.objects.all(), 1):
        group.path = Node._get_path(None, 1, i)
        group.save(using=db_alias)

[tabo]

Sorry I am late to the party, but I specifically wrote dump_bulk() and load_bulk() for this particular use case.

[mpasternak]

I'm trying to make an existing model a MP_Node and I'm planning to use this migration. Is this approach anywhere in the docs?

[bmihelac]

This article provides a step-by-step guide on how to migrate an existing model with data while preserving its structure using Django migrations: https://code.informatikamihelac.com/en/django-treebeard-converting-an-existing-model-to-mp_node/