convert DecimalBlock value back to Decimal (#8694)

Fixes #4647

CHANGELOG.txt

@@ -57,6 +57,7 @@ Changelog
  * Fix: Prevent error on sending notifications for the legacy moderation process when no user was specified (Yves Serrano)
  * Fix: Ensure `aria-label` is not set on locale selection dropdown within page chooser modal as it was a duplicate of the button contents (LB (Ben Johnston))
  * Fix: Revise the `ModelAdmin` title column behaviour to only link to 'edit' if the user has the correct permissions, fallback to the 'inspect' view or a non-clickable title if needed (Stefan Hammer)
+ * Fix: Ensure that `DecimalBlock` preserves the `Decimal` type when retrieving from the database (Yves Serrano)
 
 
 3.0.1 (16.06.2022)

docs/releases/4.0.md

@@ -70,6 +70,7 @@ When using a queryset to render a list of images, you can now use the ``prefetch
  * Prevent error on sending notifications for the legacy moderation process when no user was specified (Yves Serrano)
  * Ensure `aria-label` is not set on locale selection dropdown within page chooser modal as it was a duplicate of the button contents (LB (Ben Johnston))
  * Revise the `ModelAdmin` title column behaviour to only link to 'edit' if the user has the correct permissions, fallback to the 'inspect' view or a non-clickable title if needed (Stefan Hammer)
+ * Ensure that `DecimalBlock` preserves the `Decimal` type when retrieving from the database (Yves Serrano)
 
 
 ## Upgrade considerations

wagtail/blocks/field_block.py

@@ -1,4 +1,5 @@
 import datetime
+from decimal import Decimal
 
 from django import forms
 from django.db.models.fields import BLANK_CHOICE_DASH
@@ -245,6 +246,9 @@ class DecimalBlock(FieldBlock):
         )
         super().__init__(*args, **kwargs)
 
+    def to_python(self, value):
+        return Decimal(value)
+
     class Meta:
         icon = "plus-inverse"
 

wagtail/tests/test_blocks.py

@@ -8,6 +8,7 @@ from decimal import Decimal
 # non-standard import name for gettext_lazy, to prevent strings from being picked up for translation
 from django import forms
 from django.core.exceptions import ValidationError
+from django.core.serializers.json import DjangoJSONEncoder
 from django.forms.utils import ErrorList
 from django.template.loader import render_to_string
 from django.test import SimpleTestCase, TestCase
@@ -411,6 +412,13 @@ class TestDecimalBlock(TestCase):
         block_val = block.value_from_form(Decimal("1.63"))
         self.assertEqual(type(block_val), Decimal)
 
+    def test_type_to_python(self):
+        block = blocks.DecimalBlock()
+        block_val = block.to_python(
+            "1.63"
+        )  # decimals get saved as string in JSON field
+        self.assertEqual(type(block_val), Decimal)
+
     def test_render(self):
         block = blocks.DecimalBlock()
         test_val = Decimal(1.63)
@@ -446,6 +454,16 @@ class TestDecimalBlock(TestCase):
         with self.assertRaises(ValidationError):
             block.clean("3.0")
 
+    def test_round_trip_to_db_preserves_type(self):
+        block = blocks.DecimalBlock()
+        original_value = Decimal(1.63)
+        db_value = json.dumps(
+            block.get_prep_value(original_value), cls=DjangoJSONEncoder
+        )
+        restored_value = block.to_python(json.loads(db_value))
+        self.assertEqual(type(restored_value), Decimal)
+        self.assertEqual(original_value, restored_value)
+
 
 class TestRegexBlock(TestCase):
     def test_render(self):