✅ Clarify test for prefill from options

The plugin return value determines which values get assigned to which
variables - there is no default mechanism that stores the retrieved
object in the variable that holds the prefill configuration.

This means that for the objects API, the retrieved object is never
persisted to DB, only the mapped variables contained in the
configuration.

src/openforms/prefill/tests/test_prefill_variables.py

@@ -26,6 +26,7 @@
     SubmissionFactory,
     SubmissionStepFactory,
 )
+from openforms.variables.constants import FormVariableDataTypes
 
 from ..contrib.demo.plugin import DemoPrefill
 from ..service import prefill_variables
@@ -265,13 +266,23 @@ def get_prefill_values_from_options(cls, submission: Submission, options):
 class PrefillVariablesFromOptionsTests(TestCase):
     @patch(
         "openforms.prefill.service.fetch_prefill_values_from_options",
-        return_value={"voornamen": "Not so random string"},
+        return_value={
+            "object_data": {
+                "voornamen": "Not so random string",
+            },
+            "voornamen": "Not so random string",
+        },
     )
     def test_applying_prefill_plugin_from_user_defined_with_options(self, m_prefill):
         submission = SubmissionFactory.create()
         FormVariableFactory.create(
-            key="voornamen",
+            key="voornamen", form=submission.form, user_defined=True
+        )
+        FormVariableFactory.create(
+            key="object_data",
             form=submission.form,
+            user_defined=True,
+            data_type=FormVariableDataTypes.object,
             prefill_plugin="objects_api",
             prefill_options={
                 "objects_api_group": 1,
@@ -285,16 +296,9 @@ def test_applying_prefill_plugin_from_user_defined_with_options(self, m_prefill)
 
         prefill_variables(submission=submission)
 
-        submission_value_variables_state = (
-            submission.load_submission_value_variables_state()
-        )
-
-        self.assertEqual(1, len(submission_value_variables_state.variables))
-
-        submission_variable = submission_value_variables_state.get_variable(
-            key="voornamen"
-        )
-
+        variables_state = submission.load_submission_value_variables_state(refresh=True)
+        self.assertEqual(len(variables_state.variables), 2)
+        submission_variable = variables_state.get_variable(key="voornamen")
         self.assertEqual("Not so random string", submission_variable.value)
         self.assertEqual(
             SubmissionValueVariableSources.prefill, submission_variable.source