Handle `child_block` being passed as a kwarg in ListBlock.deconstruct_with_lookup (#12208)

Fixes #12202

CHANGELOG.txt

@@ -15,7 +15,7 @@ Changelog
 6.2.1 (xx.xx.20xx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~~~
 
- * ...
+ * Fix: Handle `child_block` being passed as a kwarg in ListBlock migrations (Matt Westcott)
 
 
 6.2 (01.08.2024)

docs/releases/6.2.1.md

@@ -14,4 +14,4 @@ depth: 1
 
 ### Bug fixes
 
- * ...
+ * Handle `child_block` being passed as a kwarg in ListBlock migrations (Matt Westcott)

wagtail/blocks/list_block.py

@@ -159,11 +159,16 @@ class ListBlock(Block):
 
     @classmethod
     def construct_from_lookup(cls, lookup, *args, **kwargs):
-        if getattr(cls.__init__, "has_child_block_arg", False) and isinstance(
-            args[0], int
-        ):
-            child_block = lookup.get_block(args[0])
-            args = (child_block, *args[1:])
+        if getattr(cls.__init__, "has_child_block_arg", False):
+            if args and isinstance(args[0], int):
+                child_block = lookup.get_block(args[0])
+                args = (child_block, *args[1:])
+            else:
+                child_block_kwarg = kwargs.get("child_block")
+                if isinstance(child_block_kwarg, int):
+                    child_block = lookup.get_block(child_block_kwarg)
+                    kwargs["child_block"] = child_block
+
         return cls(*args, **kwargs)
 
     def value_from_datadict(self, data, files, prefix):
@@ -413,11 +418,16 @@ class ListBlock(Block):
 
     def deconstruct_with_lookup(self, lookup):
         path, args, kwargs = super().deconstruct_with_lookup(lookup)
-        if getattr(self.__init__, "has_child_block_arg", False) and isinstance(
-            args[0], Block
-        ):
-            block_id = lookup.add_block(args[0])
-            args = (block_id, *args[1:])
+        if getattr(self.__init__, "has_child_block_arg", False):
+            if args and isinstance(args[0], Block):
+                block_id = lookup.add_block(args[0])
+                args = (block_id, *args[1:])
+            else:
+                child_block = kwargs.get("child_block")
+                if isinstance(child_block, Block):
+                    block_id = lookup.add_block(child_block)
+                    kwargs["child_block"] = block_id
+
         return path, args, kwargs
 
     class Meta:

wagtail/tests/test_streamfield.py

@@ -914,6 +914,41 @@ class TestDeconstructStreamFieldWithLookup(TestCase):
             },
         )
 
+    def test_deconstruct_with_listblock_with_child_block_kwarg(self):
+        field = StreamField(
+            [
+                ("heading", blocks.CharBlock(required=True)),
+                (
+                    "bullets",
+                    blocks.ListBlock(child_block=blocks.CharBlock(required=True)),
+                ),
+            ],
+            blank=True,
+        )
+        field.set_attributes_from_name("body")
+        name, path, args, kwargs = field.deconstruct()
+        self.assertEqual(name, "body")
+        self.assertEqual(path, "wagtail.fields.StreamField")
+        self.assertEqual(
+            args,
+            [
+                [
+                    ("heading", 0),
+                    ("bullets", 1),
+                ]
+            ],
+        )
+        self.assertEqual(
+            kwargs,
+            {
+                "blank": True,
+                "block_lookup": {
+                    0: ("wagtail.blocks.CharBlock", (), {"required": True}),
+                    1: ("wagtail.blocks.ListBlock", (), {"child_block": 0}),
+                },
+            },
+        )
+
     def test_deconstruct_with_listblock_subclass(self):
         # See https://github.com/wagtail/wagtail/issues/12164 - unlike StructBlock and StreamBlock,
         # ListBlock's deconstruct method doesn't reduce subclasses to the base ListBlock class.