API bug fix: Distinct query (#5940)

* Fix "company" filter for StockList

- distinct() was in the wrong spot
- Added a new unit test to cover this

* Update stocklist API filter

- Move custom filtering into FilterSet class
- Exposes available filters to API documentation
- Improved readability / field validation

* Further improvements for StockList API

* For for order extra line item serializer

- 'title' is not a valid field

InvenTree/stock/test_api.py

@@ -486,6 +486,11 @@ class StockItemListTest(StockAPITestCase):
         response = self.get_stock(batch='B123')
         self.assertEqual(len(response), 1)
 
+    def test_filter_by_company(self):
+        """Test that we can filter stock items by company"""
+        for cmp in company.models.Company.objects.all():
+            self.get_stock(company=cmp.pk)
+
     def test_filter_by_serialized(self):
         """Filter StockItem by serialized status."""
         response = self.get_stock(serialized=1)
@@ -740,10 +745,10 @@ class StockItemListTest(StockAPITestCase):
     def test_query_count(self):
         """Test that the number of queries required to fetch stock items is reasonable."""
 
-        def get_stock(data):
+        def get_stock(data, expected_status=200):
             """Helper function to fetch stock items."""
             response = self.client.get(self.list_url, data=data)
-            self.assertEqual(response.status_code, 200)
+            self.assertEqual(response.status_code, expected_status)
             return response.data
 
         # Create a bunch of StockItem objects