Convert 'part" to MPTT model - based on the 'variant_of' field

- Now recursive variants can be implemented properly

InvenTree/part/migrations/0039_auto_20200515_1127.py

@@ -0,0 +1,50 @@
+# Generated by Django 3.0.5 on 2020-05-15 11:27
+
+from django.db import migrations, models
+
+from part.models import Part
+
+
+def update_tree(apps, schema_editor):
+    # Update the MPTT for Part model
+    Part.objects.rebuild()
+
+
+def nupdate_tree(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0038_auto_20200513_0016'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='level',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='lft',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='rght',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='tree_id',
+            field=models.PositiveIntegerField(db_index=True, default=0, editable=False),
+            preserve_default=False,
+        ),
+
+        migrations.RunPython(update_tree, reverse_code=nupdate_tree)
+    ]

InvenTree/part/models.py

@@ -24,7 +24,7 @@ from markdownx.models import MarkdownxField
 
 from django_cleanup import cleanup
 
-from mptt.models import TreeForeignKey
+from mptt.models import TreeForeignKey, MPTTModel
 
 from stdimage.models import StdImageField
 
@@ -200,7 +200,7 @@ def match_part_names(match, threshold=80, reverse=True, compare_length=False):
 
 
 @cleanup.ignore
-class Part(models.Model):
+class Part(MPTTModel):
     """ The Part object represents an abstract part, the 'concept' of an actual entity.
 
     An actual physical instance of a Part is a StockItem which is treated separately.
@@ -236,8 +236,12 @@ class Part(models.Model):
     """
 
     class Meta:
-        verbose_name = "Part"
+        verbose_name = _("Part")
-        verbose_name_plural = "Parts"
+        verbose_name_plural = _("Parts")
+
+    class MPTTMeta:
+        # For legacy reasons the 'variant_of' field is used to indicate the MPTT parent
+        parent_attr='variant_of'
 
     def save(self, *args, **kwargs):
         """