Reverse lookup for part category URL

- Added some test cases for PartCategory model

InvenTree/part/test_category.py

@@ -0,0 +1,67 @@
+from django.test import TestCase
+
+from .models import Part, PartCategory
+from .models import rename_part_image
+
+
+class CategoryTest(TestCase):
+    """
+    Tests to ensure that the relational category tree functions correctly.
+    """
+
+    def setUp(self):
+        self.p1 = PartCategory.objects.create(name='A',
+                                         description='Most highest level',
+                                         parent=None)
+
+        self.p2 = PartCategory.objects.create(name='B',
+                                         description='Sits under second',
+                                         parent=self.p1)
+
+        self.p3 = PartCategory.objects.create(name='C',
+                                         description='Third tier category',
+                                         parent=self.p2)
+
+        # Add two parts in p2
+        Part.objects.create(name='Flange', category=self.p2)
+        Part.objects.create(name='Flob', category=self.p2)
+
+        # Add one part in p3
+        Part.objects.create(name='Blob', category=self.p3)
+
+    def test_parents(self):
+        self.assertEqual(self.p1.parent, None)
+        self.assertEqual(self.p2.parent, self.p1)
+        self.assertEqual(self.p3.parent, self.p2)
+
+    def test_children_count(self):
+        self.assertEqual(self.p1.has_children, True)
+        self.assertEqual(self.p2.has_children, True)
+        self.assertEqual(self.p3.has_children, False)
+
+    def test_unique_childs(self):
+        childs = self.p1.getUniqueChildren()
+
+        self.assertIn(self.p2.id, childs)
+        self.assertIn(self.p2.id, childs)
+
+    def test_unique_parents(self):
+        parents = self.p2.getUniqueParents()
+
+        self.assertIn(self.p1.id, parents)
+
+    def test_path_string(self):
+        self.assertEqual(str(self.p3), 'A/B/C')
+
+    def test_url(self):
+        self.assertEqual(self.p1.get_absolute_url(), '/part/category/1/')
+
+    def test_part_count(self):
+        # No direct parts in the top-level category
+        self.assertEqual(self.p1.has_parts, False)
+        self.assertEqual(self.p2.has_parts, True)
+        self.assertEqual(self.p3.has_parts, True)
+
+        self.assertEqual(self.p1.partcount, 3)
+        self.assertEqual(self.p2.partcount, 3)
+        self.assertEqual(self.p3.partcount, 1)