Compare StreamField blocks by serialised value when checking has_changed for a form, preventing form.has_changed from always returning True for pages with StreamFields

wagtail/core/blocks/base.py

@@ -542,6 +542,9 @@ class BlockField(forms.Field):
     def clean(self, value):
         return self.block.clean(value)
 
+    def has_changed(self, initial_value, data_value):
+        return self.block.get_prep_value(initial_value) != self.block.get_prep_value(data_value)
+
 
 DECONSTRUCT_ALIASES = {
     Block: 'wagtail.core.blocks.Block',

wagtail/core/tests/test_blocks.py

@@ -2417,6 +2417,23 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
         self.assertEqual(list(block.child_blocks.keys()),
                          ['heading', 'paragraph', 'intro', 'by_line'])
 
+    def test_field_has_changed(self):
+        block = blocks.StreamBlock([('paragraph', blocks.CharBlock())])
+        initial_value = blocks.StreamValue(block, [('paragraph', 'test')])
+        initial_value[0].id = 'a'
+
+        data_value = blocks.StreamValue(block, [('paragraph', 'test')])
+        data_value[0].id = 'a'
+
+        # identical ids and content, so has_changed should return False
+        self.assertFalse(blocks.BlockField(block).has_changed(initial_value, data_value))
+
+        changed_data_value = blocks.StreamValue(block, [('paragraph', 'not a test')])
+        changed_data_value[0].id = 'a'
+
+        # identical ids but changed content, so has_changed should return True
+        self.assertTrue(blocks.BlockField(block).has_changed(initial_value, changed_data_value))
+
     def test_required_raises_an_exception_if_empty(self):
         block = blocks.StreamBlock([('paragraph', blocks.CharBlock())], required=True)
         value = blocks.StreamValue(block, [])