Add custom migration

- Required to initialize the MPTT fields for the StockItem model

InvenTree/stock/admin.py

@@ -85,6 +85,13 @@ class StockItemResource(ModelResource):
     
     stocktake_date = Field(attribute='stocktake_date', widget=widgets.DateWidget())
 
+    def after_import(self, dataset, result, using_transactions, dry_run, **kwargs):
+
+        super().after_import(dataset, result, using_transactions, dry_run, **kwargs)
+
+        # Rebuild the StockItem tree(s)
+        StockItem.objects.rebuild()
+
     class Meta:
         model = StockItem
         skip_unchanged = True

InvenTree/stock/migrations/0022_auto_20200217_1109.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.9 on 2020-02-17 11:09
+
+from django.db import migrations
+from stock import models
+
+
+def update_stock_item_tree(apps, schema_editor):
+    # Update the StockItem MPTT model
+
+    models.StockItem.objects.rebuild()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0021_auto_20200215_2232'),
+    ]
+
+    operations = [
+        migrations.RunPython(update_stock_item_tree)
+    ]

InvenTree/stock/models.py

@@ -385,6 +385,13 @@ class StockItem(MPTTModel):
 
         return True
 
+    @property
+    def child_count(self):
+        """ Return the number of 'child' items associated with this StockItem.
+        A child item is one which has been split from this one.
+        """
+        return self.get_descendants(include_self=False).count()
+
     @property
     def in_stock(self):