fix: give an answerless legacy choice question an empty choice interaction

Every newly added question is one: the editor writes type single_selection
with no answers, and the XSD requires at least one qti-simple-choice, so
conversion raised. Stand in the single empty choice the QTI editor opens a
new choice interaction with.

Author: rtibblesbot

contentcuration/contentcuration/tests/utils/qti/fixtures/single_selection_no_answers.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<qti-assessment-item xmlns="http://www.imsglobal.org/xsd/imsqtiasi_v3p0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.imsglobal.org/xsd/imsqtiasi_v3p0 https://purl.imsglobal.org/spec/qti/v3p0/schema/xsd/imsqti_asiv3p0p1_v1p0.xsd" identifier="Kq83vEjRWeJCrze8SNFZ4kA" title="Test Question 1" adaptive="false" time-dependent="false" xml:lang="en-US" tool-name="kolibri" tool-version="0.1">
+    <qti-response-declaration identifier="RESPONSE" cardinality="single" base-type="identifier" />
+    <qti-outcome-declaration identifier="SCORE" cardinality="single" base-type="float" />
+    <qti-item-body>
+        <qti-choice-interaction response-identifier="RESPONSE" shuffle="true" max-choices="1" min-choices="0" orientation="vertical">
+            <qti-prompt>
+                <p>What is 2+2?</p>
+            </qti-prompt>
+            <qti-simple-choice identifier="choice_0" show-hide="show" fixed="false" />
+        </qti-choice-interaction>
+    </qti-item-body>
+    <qti-response-processing template="https://purl.imsglobal.org/spec/qti/v3p0/rptemplates/match_correct" />
+</qti-assessment-item>

contentcuration/contentcuration/tests/utils/qti/test_convert.py

@@ -117,6 +117,57 @@ def test_true_false(self):
         )
         self.assertTrue(validate_qti_item(result.xml.encode("utf-8")).is_valid)
 
+    def test_single_selection_no_answers(self):
+        item = _make_item(
+            type=exercises.SINGLE_SELECTION,
+            question="What is 2+2?",
+            answers=[],
+            randomize=True,
+            assessment_id="abcdef1234567890abcdef1234567890",
+        )
+
+        result = convert_legacy_assessment_item_to_qti(item)
+
+        self.assertEqual(result.identifier, "Kq83vEjRWeJCrze8SNFZ4kA")
+        self.assertEqual(
+            _normalize_xml(_load_fixture("single_selection_no_answers.xml")),
+            _normalize_xml(result.xml),
+        )
+        self.assertTrue(validate_qti_item(result.xml.encode("utf-8")).is_valid)
+
+    def test_choice_types_with_no_answers_get_one_empty_choice(self):
+        # test_single_selection_no_answers pins SINGLE_SELECTION against the fixture.
+        for question_type in (exercises.MULTIPLE_SELECTION, "true_false"):
+            with self.subTest(question_type=question_type):
+                item = _make_item(
+                    type=question_type,
+                    question="What is 2+2?",
+                    answers=[],
+                    assessment_id="abcdef1234567890abcdef1234567890",
+                )
+
+                result = convert_legacy_assessment_item_to_qti(item)
+
+                self.assertIn(
+                    '<qti-simple-choice identifier="choice_0" show-hide="show" fixed="false" />',
+                    result.xml,
+                )
+                self.assertIn("<p>What is 2+2?</p>", result.xml)
+                self.assertTrue(validate_qti_item(result.xml.encode("utf-8")).is_valid)
+
+    def test_choice_type_with_no_answers_and_no_question(self):
+        item = _make_item(
+            type=exercises.MULTIPLE_SELECTION,
+            question="",
+            answers=[],
+            assessment_id="abcdef1234567890abcdef1234567890",
+        )
+
+        result = convert_legacy_assessment_item_to_qti(item)
+
+        self.assertIn("<qti-prompt />", result.xml)
+        self.assertTrue(validate_qti_item(result.xml.encode("utf-8")).is_valid)
+
     def test_media_reference_survives(self):
         item = _make_item(
             type=exercises.SINGLE_SELECTION,

contentcuration/contentcuration/utils/assessment/qti/convert.py

@@ -164,9 +164,14 @@ def _create_choice_interaction_and_response(
 
     prompt = Prompt(children=_create_html_content_from_text(item.question))
 
+    # Every newly added question starts answerless, but the XSD requires at least one
+    # qti-simple-choice. Stand in the single empty choice the QTI editor opens a new
+    # choice interaction with.
+    answers = item.answers or [{}]
+
     choices = []
     correct_values = []
-    for i, answer in enumerate(item.answers):
+    for i, answer in enumerate(answers):
         choice_id = f"choice_{i}"
         choice_content = _create_html_content_from_text(answer.get("answer", ""))