Ensure StructBlock.get_default properly reflects child block defaults

StructBlock previously handled default values 'lazily' at render time, setting meta.default to fall back to an empty dict and calling the child block's get_default on missing items. We can't do this with client-side rendering because get_default isn't available on the client-side block definition (and adding it to the client-side block API would introduce unnecessary bloat). Instead, we fix StructBlock.get_default to correctly pick up the defaults from its child blocks, so that anything that makes use of StructBlock.get_default (e.g. a containing ListBlock / StreamBlock that needs to create StructBlock children on the fly) will have the full accurate data to hand.

wagtail/core/blocks/struct_block.py

@@ -54,7 +54,13 @@ class BaseStructBlock(Block):
         rather than a StructValue; for consistency, we need to convert it to a StructValue
         for StructBlock to work with
         """
-        return self._to_struct_value(self.meta.default.items())
+        return self._to_struct_value([
+            (
+                name,
+                self.meta.default[name] if name in self.meta.default else block.get_default()
+            )
+            for name, block in self.child_blocks.items()
+        ])
 
     def value_from_datadict(self, data, files, prefix):
         return self._to_struct_value([

wagtail/core/tests/test_blocks.py

@@ -1611,6 +1611,15 @@ class TestStructBlock(SimpleTestCase):
             html
         )
 
+    def test_get_default(self):
+        class LinkBlock(blocks.StructBlock):
+            title = blocks.CharBlock(default="Torchbox")
+            link = blocks.URLBlock(default="http://www.torchbox.com")
+
+        block = LinkBlock()
+        default_val = block.get_default()
+        self.assertEqual(default_val.get('title'), 'Torchbox')
+
     def test_render_form_with_help_text(self):
         class LinkBlock(blocks.StructBlock):
             title = blocks.CharBlock()