Fix empty blocks created in migration operations

CHANGELOG.txt

@@ -30,6 +30,7 @@ Changelog
  * Fix: Use constant-time comparison for image serve URL signatures (Jake Howard)
  * Fix: Ensure taggit field type-ahead options show correctly in the dark mode theme (Sage Abdullah)
  * Fix: Fix the lock description message missing the model_name variable when locked only by system (Sébastien Corbin)
+ * Fix: Fix empty blocks created in migration operations (Sandil Ranasinghe)
  * Docs: Document how to add non-ModelAdmin views to a `ModelAdminGroup` (Onno Timmerman)
  * Docs: Document how to add StructBlock data to a StreamField (Ramon Wenger)
  * Docs: Update ReadTheDocs settings to v2 to resolve urllib3 issue in linkcheck extension (Thibaud Colas)

docs/releases/5.1.md

@@ -59,6 +59,7 @@ Thank you to Damilola for his work, and to Google for sponsoring this project.
  * Use constant-time comparison for image serve URL signatures (Jake Howard)
  * Ensure taggit field type-ahead options show correctly in the dark mode theme (Sage Abdullah)
  * Fix the lock description message missing the model_name variable when locked only by system (Sébastien Corbin)
+ * Fix empty blocks created in migration operations (Sandil Ranasinghe)
 
 ### Documentation
 

wagtail/blocks/migrations/operations.py

@@ -164,8 +164,11 @@ class StreamChildrenToListBlockOperation(BaseBlockOperation):
                 mapped_block_value.append(child_block)
 
         self.map_temp_blocks_to_list_items()
-        new_list_block = {"type": self.list_block_name, "value": self.temp_blocks}
-        mapped_block_value.append(new_list_block)
+
+        if self.temp_blocks:
+            new_list_block = {"type": self.list_block_name, "value": self.temp_blocks}
+            mapped_block_value.append(new_list_block)
+
         return mapped_block_value
 
     def map_temp_blocks_to_list_items(self):
@@ -206,8 +209,10 @@ class StreamChildrenToStreamBlockOperation(BaseBlockOperation):
             else:
                 mapped_block_value.append(child_block)
 
-        new_stream_block = {"type": self.stream_block_name, "value": stream_value}
-        mapped_block_value.append(new_stream_block)
+        if stream_value:
+            new_stream_block = {"type": self.stream_block_name, "value": stream_value}
+            mapped_block_value.append(new_stream_block)
+
         return mapped_block_value
 
     @property

wagtail/tests/streamfield_migrations/test_simple_structures.py

@@ -107,6 +107,27 @@ class FieldChildBlockTest(TestCase):
             altered_raw_data[2]["value"][1]["value"], self.raw_data[2]["value"]
         )
 
+    def test_combine_to_listblock_no_existing_children(self):
+        """Combine all `simplestruct` blocks into a new ListBlock named `list1`
+        We have no `simplestruct` blocks in our existing data, so there should be no list1 blocks
+        created and the data should be intact.
+        """
+
+        altered_raw_data = apply_changes_to_raw_data(
+            raw_data=self.raw_data,
+            block_path_str="",
+            operation=StreamChildrenToListBlockOperation(
+                block_name="simplestruct", list_block_name="list1"
+            ),
+            streamfield=models.SampleModel.content,
+        )
+
+        self.assertEqual(len(altered_raw_data), 4)
+        self.assertEqual(altered_raw_data[0], self.raw_data[0])
+        self.assertEqual(altered_raw_data[1], self.raw_data[1])
+        self.assertEqual(altered_raw_data[2], self.raw_data[2])
+        self.assertEqual(altered_raw_data[3], self.raw_data[3])
+
     def test_combine_single_type_to_streamblock(self):
         """Combine all `char1` blocks as children of a new StreamBlock named `stream1`
 
@@ -166,6 +187,27 @@ class FieldChildBlockTest(TestCase):
         self.assertEqual(altered_raw_data[0]["value"][2], self.raw_data[2])
         self.assertEqual(altered_raw_data[0]["value"][3], self.raw_data[3])
 
+    def test_combine_to_streamblock_no_existing_children(self):
+        """Combine all `simplestruct` blocks as children of a new StreamBlock named `stream1`
+        We have no `simplestruct` blocks in our existing data, so there should be no stream1 blocks
+        created and the data should be intact.
+        """
+
+        altered_raw_data = apply_changes_to_raw_data(
+            raw_data=self.raw_data,
+            block_path_str="",
+            operation=StreamChildrenToStreamBlockOperation(
+                block_names=["simplestruct"], stream_block_name="stream1"
+            ),
+            streamfield=models.SampleModel.content,
+        )
+
+        self.assertEqual(len(altered_raw_data), 4)
+        self.assertEqual(altered_raw_data[0], self.raw_data[0])
+        self.assertEqual(altered_raw_data[1], self.raw_data[1])
+        self.assertEqual(altered_raw_data[2], self.raw_data[2])
+        self.assertEqual(altered_raw_data[3], self.raw_data[3])
+
     def test_to_structblock(self):
         """Move each `char1` block inside a new StructBlock named `struct1`