Skip length validators on Python Enum columns

CHANGELOG.rst

@@ -4,6 +4,12 @@ Changelog
 unreleased
 ++++++++++
 
+Bug fixes:
+
+* Don't attach a length validator to Python ``Enum`` columns. Validators run
+  after deserialization, so ``len()`` was called on the enum member
+  (:issue:`673`).
+
 Other changes:
 
 * Drop support for marshmallow 3, which is EOL.

src/marshmallow_sqlalchemy/convert.py

@@ -445,7 +445,11 @@ def _add_column_kwargs(self, kwargs: dict[str, Any], column: sa.Column) -> None:
                 except (AttributeError, NotImplementedError):
                     python_type = None
                 if not python_type or not issubclass(python_type, uuid.UUID):
-                    kwargs["validate"].append(validate.Length(max=column_length))
+                    # Enum members have no len(); the DB length is redundant
+                    # once the value is deserialized to the enum class.
+                    # https://github.com/marshmallow-code/marshmallow-sqlalchemy/issues/673
+                    if getattr(column.type, "enum_class", None) is None:
+                        kwargs["validate"].append(validate.Length(max=column_length))
 
         if getattr(column.type, "asdecimal", False):
             kwargs["places"] = getattr(column.type, "scale", None)

tests/test_conversion.py

@@ -82,7 +82,9 @@ def test_enum_with_class_converted_to_enum_field(self, models):
         field = fields_["level_with_enum_class"]
         assert type(field) is fields.Enum
         assert contains_validator(field, validate.OneOf) is False
+        assert contains_validator(field, validate.Length) is False
         assert field.enum is CourseLevel
+        assert field.deserialize("PRIMARY") is CourseLevel.PRIMARY
 
     def test_many_to_many_relationship(self, models):
         student_fields = fields_for_model(models.Student, include_relationships=True)