Make Build object into a MPTT tree model

- Builds can have child builds

InvenTree/build/migrations/0013_auto_20200425_0507.py

@@ -0,0 +1,55 @@
+# Generated by Django 3.0.5 on 2020-04-25 05:07
+
+from django.db import migrations, models
+import django.db.models.deletion
+import mptt.fields
+from build.models import Build
+
+
+def update_tree(apps, schema_editor):
+    # Update the Build MPTT model
+    Build.objects.rebuild()
+
+
+def nupdate_tree(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0012_build_sales_order'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='level',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='build',
+            name='lft',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='build',
+            name='parent',
+            field=mptt.fields.TreeForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.DO_NOTHING, related_name='children', to='build.Build'),
+        ),
+        migrations.AddField(
+            model_name='build',
+            name='rght',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='build',
+            name='tree_id',
+            field=models.PositiveIntegerField(db_index=True, default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.RunPython(update_tree, reverse_code=nupdate_tree),
+    ]