Add a better error message when a Func is missing as_mql()

Author: timgraham

django_mongodb_backend/functions.py

@@ -102,6 +102,8 @@ def extract(self, compiler, connection):
 
 def func(self, compiler, connection):
     lhs_mql = process_lhs(self, compiler, connection)
+    if self.function is None:
+        raise NotSupportedError(f"{self} may need an as_mql() method.")
     operator = MONGO_OPERATORS.get(self.__class__, self.function.lower())
     return {f"${operator}": lhs_mql}
 

tests/db_functions_/__init__.py

@@ -0,0 +1,10 @@
+from django.db import NotSupportedError
+from django.db.models import Func
+from django.test import SimpleTestCase
+
+
+class FuncTests(SimpleTestCase):
+    def test_no_as_mql(self):
+        msg = "Func() may need an as_mql() method."
+        with self.assertRaisesMessage(NotSupportedError, msg):
+            Func().as_mql(None, None)