Add 'has_variants' and 'variant_of' field for Part

- StockItem cannot point to a part which is a template part

InvenTree/part/migrations/0003_auto_20190525_2226.py

@@ -0,0 +1,24 @@
+# Generated by Django 2.2 on 2019-05-25 12:26
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0002_auto_20190520_2204'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='has_variants',
+            field=models.BooleanField(default=False, help_text='Is this part a template part?'),
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='variant_of',
+            field=models.ForeignKey(blank=True, help_text='Is this part a variant of another part?', limit_choices_to={'active': True, 'has_variants': True}, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='variants', to='part.Part'),
+        ),
+    ]

InvenTree/part/models.py

@@ -192,6 +192,7 @@ class Part(models.Model):
         description: Longer form description of the part
         keywords: Optional keywords for improving part search results
         IPN: Internal part number (optional)
+        has_variants: If True, this part is a 'template' part and cannot be instantiated as a StockItem
         URL: Link to an external page with more information about this part (e.g. internal Wiki)
         image: Image of this part
         default_location: Where the item is normally stored (may be null)
@@ -258,6 +259,17 @@ class Part(models.Model):
 
     variant = models.CharField(max_length=32, blank=True, help_text='Part variant or revision code')
 
+    has_variants = models.BooleanField(default=False, help_text='Is this part a template part?')
+
+    variant_of = models.ForeignKey('part.Part', related_name='variants',
+                                   null=True, blank=True,
+                                   limit_choices_to={
+                                       'has_variants': True,
+                                       'active': True,
+                                   },
+                                   on_delete=models.SET_NULL,
+                                   help_text='Is this part a variant of another part?')
+
     description = models.CharField(max_length=250, blank=False, help_text='Part description')
 
     keywords = models.CharField(max_length=250, blank=True, help_text='Part keywords to improve visibility in search results')