fields: fix import field validation attribute from xml (#35488)

tests/test_formdef_import.py

@@ -509,6 +509,32 @@ def test_field_condition():
     assert len(f2.fields) == len(formdef.fields)
     assert f2.fields[0].condition == {'type': 'django', 'value': '1'}
 
+def test_field_validation():
+    formdef = FormDef()
+    formdef.name = 'Foo'
+    formdef.fields = [
+        fields.StringField(type='string', id=1, label='Bar', size='40', validation={})
+        ]
+    f2 = assert_xml_import_export_works(formdef)
+    assert len(f2.fields) == len(formdef.fields)
+    assert not f2.fields[0].validation
+
+    formdef.fields = [
+        fields.StringField(type='string', id=1, label='Bar', size='40', validation=None)
+        ]
+    formdef_xml = formdef.export_to_xml()
+    f2 = FormDef.import_from_xml_tree(formdef_xml)
+    assert len(f2.fields) == len(formdef.fields)
+    assert not f2.fields[0].validation
+
+    formdef.fields = [
+        fields.StringField(type='string', id=1, label='Bar', size='40',
+            validation={'type': 'regex', 'value': r'\d'})
+        ]
+    f2 = assert_xml_import_export_works(formdef)
+    assert len(f2.fields) == len(formdef.fields)
+    assert f2.fields[0].validation == formdef.fields[0].validation
+
 def test_digest_template():
     formdef = FormDef()
     formdef.name = 'Foo'

wcs/fields.py

@@ -672,7 +672,7 @@ class StringField(WidgetField):
     widget_class = WcsExtraStringWidget
     size = None
     extra_attributes = ['size']
-    validation = None
+    validation = {}
     data_source = {}
 
     def perform_more_widget_changes(self, form, kwargs, edit=True):