✨(backend) add django-treebeard to allow tree structure on documents

We choose to use Django-treebeard for its quality, performance and
stability. Adding tree structure to documents is as simple as
inheriting from the MP_Node class.

src/backend/core/migrations/0014_add_tree_structure_to_documents.py

@@ -0,0 +1,31 @@
+# Generated by Django 5.1.2 on 2024-12-07 09:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0013_activate_fuzzystrmatch_extension'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='document',
+            name='depth',
+            field=models.PositiveIntegerField(default=0),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='document',
+            name='numchild',
+            field=models.PositiveIntegerField(default=0),
+        ),
+        migrations.AddField(
+            model_name='document',
+            name='path',
+            # Allow null values pending the next datamigration to populate the field
+            field=models.CharField(db_collation='C', max_length=252, null=True, unique=True),
+            preserve_default=False,
+        ),
+    ]

src/backend/core/migrations/0015_set_path_on_existing_documents.py

@@ -0,0 +1,52 @@
+# Generated by Django 5.1.2 on 2024-12-07 10:33
+
+from django.db import migrations, models
+
+from treebeard.numconv import NumConv
+
+ALPHABET = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+STEPLEN = 7
+
+def set_path_on_existing_documents(apps, schema_editor):
+    """
+    Updates the `path` and `depth` fields for all existing Document records 
+    to ensure valid materialized paths.
+
+    This function assigns a unique `path` to each Document as a root node
+
+    Note: After running this migration, we quickly modify the schema to make
+    the `path` field required as it should.
+    """
+    Document = apps.get_model("core", "Document")
+
+    # Iterate over all existing documents and make them root nodes
+    documents = Document.objects.order_by("created_at").values_list("id", flat=True)
+    numconv = NumConv(len(ALPHABET), ALPHABET)
+
+    updates = []
+    for i, pk in enumerate(documents):
+        key = numconv.int2str(i)
+        path = "{0}{1}".format(
+            ALPHABET[0] * (STEPLEN - len(key)),
+            key
+        )
+        updates.append(Document(pk=pk, path=path, depth=1))
+
+    # Bulk update using the prepared updates list
+    Document.objects.bulk_update(updates, ['depth', 'path'])
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0014_add_tree_structure_to_documents'),
+    ]
+
+    operations = [
+        migrations.RunPython(set_path_on_existing_documents, reverse_code=migrations.RunPython.noop),
+        migrations.AlterField(
+            model_name='document',
+            name='path',
+            field=models.CharField(db_collation='C', max_length=252, unique=True),
+        ),
+    ]