Introduce Field.clone method (#452)

The fact event generation logic was considering attributes from elements directly
and constructing new value-object based event classes directly. This does not work
well because the same attribute is shared between the actual element and the newly
generated event class.

This commit introduces a clone method at the field level so we can copy the field
safely.

src/protean/core/aggregate.py

@@ -157,7 +157,7 @@ def element_to_fact_event(element_cls):
     # Gather all fields defined in the element, except References.
     #   We ignore references in event payloads.
     attrs = {
-        key: value
+        key: value._clone()
         for key, value in fields(element_cls).items()
         if not isinstance(value, Reference)
     }

src/protean/fields/association.py

@@ -354,6 +354,14 @@ def get_cache_name(self):
     def has_changed(self):
         return self.change is not None
 
+    def _clone(self) -> "Association":
+        """
+        Clone the field with all its attributes.
+
+        :return: Cloned Field object
+        """
+        return self
+
 
 class HasOne(Association):
     """

src/protean/fields/base.py

@@ -216,6 +216,24 @@ def _run_validators(self, value):
         if errors:
             raise exceptions.ValidationError(errors)
 
+    def _clone(self) -> "Field":
+        """
+        Clone the field with all its attributes.
+
+        :return: Cloned Field object
+        """
+        return self.__class__(
+            referenced_as=self.referenced_as,
+            description=self.description,
+            identifier=self.identifier,
+            default=self.default,
+            required=self.required,
+            unique=self.unique,
+            choices=self.choices,
+            validators=self._validators,
+            error_messages=self.error_messages,
+        )
+
     def _load(self, value: Any):
         """
         Load the value for the field, run validators and return the value.

tests/field/test_clone.py

@@ -0,0 +1,105 @@
+from unittest.mock import MagicMock
+
+from protean.fields import Text
+
+
+class TestFieldClone:
+    def test_clone_basic_field(self):
+        # Arrange
+        field = Text(
+            referenced_as="test_field",
+            description="Test Text",
+            identifier=True,
+            default="default_value",
+            required=True,
+            unique=True,
+            choices=None,
+            validators=[],
+            error_messages={"invalid": "Invalid value"},
+        )
+
+        # Act
+        cloned_field = field._clone()
+
+        # Assert
+        assert cloned_field is not field, "The cloned field should be a new instance"
+        assert cloned_field.referenced_as == field.referenced_as
+        assert cloned_field.description == field.description
+        assert cloned_field.identifier == field.identifier
+        assert cloned_field.default == field.default
+        assert cloned_field.required == field.required
+        assert cloned_field.unique == field.unique
+        assert cloned_field.choices == field.choices
+        assert cloned_field.validators == field.validators
+        assert cloned_field.error_messages == field.error_messages
+
+    def test_clone_with_choices(self):
+        # Arrange
+        choices_mock = MagicMock()
+        field = Text(
+            referenced_as="test_field",
+            description="Test Text",
+            identifier=False,
+            default=None,
+            required=False,
+            unique=False,
+            choices=choices_mock,
+            validators=[],
+            error_messages={"invalid_choice": "Invalid choice"},
+        )
+
+        # Act
+        cloned_field = field._clone()
+
+        # Assert
+        assert cloned_field is not field, "The cloned field should be a new instance"
+        assert (
+            cloned_field.choices == field.choices
+        ), "Choices should be identical in the clone"
+
+    def test_clone_with_validators(self):
+        # Arrange
+        validators = [lambda x: x > 0]
+        field = Text(
+            referenced_as="test_field",
+            description="Test Text",
+            identifier=False,
+            default=None,
+            required=False,
+            unique=False,
+            choices=None,
+            validators=validators,
+            error_messages={"required": "This field is required"},
+        )
+
+        # Act
+        cloned_field = field._clone()
+
+        # Assert
+        assert cloned_field is not field, "The cloned field should be a new instance"
+        assert (
+            cloned_field.validators == field.validators
+        ), "Validators should be identical in the clone"
+
+    def test_clone_with_default_callable(self):
+        # Arrange
+        field = Text(
+            referenced_as="test_field",
+            description="Test Text",
+            identifier=False,
+            default=lambda: "dynamic_default",
+            required=False,
+            unique=False,
+            choices=None,
+            validators=[],
+            error_messages={"invalid": "Invalid value"},
+        )
+
+        # Act
+        cloned_field = field._clone()
+
+        # Assert
+        assert cloned_field is not field, "The cloned field should be a new instance"
+        assert (
+            cloned_field.default is field.default
+        ), "Default callable should be identical in the clone"