Add 'parent' field for StockItem

- Allows StockItem to be tracked when it is split into multiple items
- Uses MPTT field

InvenTree/stock/migrations/0021_auto_20200215_2232.py

@@ -0,0 +1,44 @@
+# Generated by Django 2.2.9 on 2020-02-15 22:32
+
+from django.db import migrations, models
+import django.db.models.deletion
+import mptt.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0020_auto_20200206_1213'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitem',
+            name='level',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='stockitem',
+            name='lft',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='stockitem',
+            name='parent',
+            field=mptt.fields.TreeForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.DO_NOTHING, related_name='children', to='stock.StockItem'),
+        ),
+        migrations.AddField(
+            model_name='stockitem',
+            name='rght',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='stockitem',
+            name='tree_id',
+            field=models.PositiveIntegerField(db_index=True, default=0, editable=False),
+            preserve_default=False,
+        ),
+    ]