Fix StreamValue._prefetch_blocks() to skip manually inserted items

Fixes #12320

wagtail/blocks/stream_block.py

@@ -653,7 +653,7 @@ class StreamValue(MutableSequence):
         raw_values = OrderedDict(
             (i, raw_item["value"])
             for i, raw_item in enumerate(self._raw_data)
-            if raw_item["type"] == type_name and self._bound_blocks[i] is None
+            if self._bound_blocks[i] is None and raw_item["type"] == type_name
         )
         # pass the raw block values to bulk_to_python as a list
         converted_values = child_block.bulk_to_python(raw_values.values())

wagtail/tests/test_streamfield.py

@@ -237,6 +237,24 @@ class TestStreamValueAccess(TestCase):
         self.assertEqual(fetched_body[1].block_type, "text")
         self.assertEqual(fetched_body[1].value, "bar")
 
+    def test_can_append_on_queried_instance(self):
+        # The test is analog to test_can_append(), but instead of working with the
+        # in-memory version from JSONStreamModel.objects.create(), we query a fresh
+        # instance from the db.
+        # It tests adding data to child blocks that
+        # have not yet been lazy loaded. This would previously crash.
+        self.json_body = JSONStreamModel.objects.get(pk=self.json_body.pk)
+        self.json_body.body.append(("text", "bar"))
+        self.json_body.save()
+
+        fetched_body = JSONStreamModel.objects.get(id=self.json_body.id).body
+        self.assertIsInstance(fetched_body, StreamValue)
+        self.assertEqual(len(fetched_body), 2)
+        self.assertEqual(fetched_body[0].block_type, "text")
+        self.assertEqual(fetched_body[0].value, "foo")
+        self.assertEqual(fetched_body[1].block_type, "text")
+        self.assertEqual(fetched_body[1].value, "bar")
+
 
 class TestStreamFieldRenderingBase(TestCase):
     model = JSONStreamModel