Allow role assignment queries to use .only (#482)

Fixes #449

ansible_base/rbac/models.py

@@ -389,14 +389,14 @@ class Meta:
         app_label = 'dab_rbac'
         abstract = True
 
-    def __init__(self, *args, **kwargs):
-        super().__init__(*args, **kwargs)
-
-        # Cache fields from the associated object_role
-        if self.object_role_id and not self.object_id:
-            self.object_id = self.object_role.object_id
-            self.content_type_id = self.object_role.content_type_id
-            self.role_definition_id = self.object_role.role_definition_id
+    def save(self, *args, **kwargs):
+        if not self.id:  # usually only new objects can be saved, but super needs to do error handling
+            if self.object_role_id and (not self.object_id):
+                # Cache fields from the associated object_role
+                self.object_id = self.object_role.object_id
+                self.content_type_id = self.object_role.content_type_id
+                self.role_definition_id = self.object_role.role_definition_id
+        return super().save(*args, **kwargs)
 
 
 class RoleUserAssignment(AssignmentBase):

test_app/tests/rbac/models/test_role_assignments.py

@@ -0,0 +1,17 @@
+import pytest
+
+from ansible_base.rbac.models import RoleUserAssignment
+
+
+@pytest.mark.django_db
+def test_load_assignment_list(rando, inventory, inv_rd, global_inv_rd):
+    assignment = inv_rd.give_permission(rando, inventory)
+    global_inv_rd.give_global_permission(rando)
+    assert assignment.id in [asmt.id for asmt in RoleUserAssignment.objects.only('id')]
+
+
+@pytest.mark.django_db
+def test_load_assignment_property(rando, inventory, inv_rd, global_inv_rd):
+    assignment = inv_rd.give_permission(rando, inventory)
+    global_inv_rd.give_global_permission(rando)
+    assert str(assignment.object_id) in [asmt.object_id for asmt in RoleUserAssignment.objects.only('object_id')]