diff --git a/optuna_dashboard/_preferential_history.py b/optuna_dashboard/_preferential_history.py
new file mode 100644
index 00000000..df203f1a
--- /dev/null
+++ b/optuna_dashboard/_preferential_history.py
@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+from datetime import datetime
+import json
+from typing import TypedDict
+import uuid
+
+from optuna.storages import BaseStorage
+
+from .preferential._system_attrs import report_preferences
+
+
+_SYSTEM_ATTR_PREFIX_HISTORY = "preference:history"
+
+
+class Choice(TypedDict):
+    uuid: str
+    candidate_trials: list[int]
+    preference_uuid: str
+    timestamp: datetime
+
+
+def report_choice(
+    study_id: int,
+    storage: BaseStorage,
+    candidate_trials: list[int],
+    preferences: list[tuple[int, int]],
+    timestamp: datetime,
+) -> None:
+    choice: Choice = {
+        "uuid": str(uuid.uuid4()),
+        "candidate_trials": candidate_trials,
+        "preference_uuid": report_preferences(study_id, storage, preferences),
+        "timestamp": timestamp,
+    }
+    key = _SYSTEM_ATTR_PREFIX_HISTORY + choice["uuid"]
+    storage.set_study_system_attr(
+        study_id=study_id,
+        key=key,
+        value=json.dumps(choice),
+    )
diff --git a/optuna_dashboard/preferential/_history.py b/optuna_dashboard/preferential/_history.py
deleted file mode 100644
index fba0c69d..00000000
--- a/optuna_dashboard/preferential/_history.py
+++ /dev/null
@@ -1,65 +0,0 @@
-from __future__ import annotations
-
-from dataclasses import asdict
-from dataclasses import dataclass
-from datetime import datetime
-from enum import Enum
-import json
-from json import JSONEncoder
-from typing import Any
-import uuid
-
-from optuna.storages import BaseStorage
-
-from ._system_attrs import report_preferences
-
-
-_SYSTEM_ATTR_PREFIX_HISTORY = "preference:history"
-
-
-class FeedbackMode(Enum):
-    CHOOSE_WORST = 0
-    AUTO = 10
-
-
-@dataclass
-class Choice:
-    uuid: str
-    candidate_trials: list[int]
-    preference_uuid: str
-    feedback_mode: FeedbackMode
-    timestamp: datetime
-
-
-class Encoder(JSONEncoder):
-    def default(self, a: Any) -> Any:
-        if isinstance(a, FeedbackMode):
-            return a.name
-        if isinstance(a, Choice):
-            return asdict(a)
-        if isinstance(a, datetime):
-            return a.isoformat()
-        return super().default(a)
-
-
-def report_choice(
-    study_id: int,
-    storage: BaseStorage,
-    candidate_trials: list[int],
-    preferences: list[tuple[int, int]],
-    feedback_mode: FeedbackMode,
-    timestamp: datetime,
-) -> None:
-    choice = Choice(
-        uuid=str(uuid.uuid4()),
-        candidate_trials=candidate_trials,
-        preference_uuid=report_preferences(study_id, storage, preferences),
-        feedback_mode=feedback_mode,
-        timestamp=timestamp,
-    )
-    key = _SYSTEM_ATTR_PREFIX_HISTORY + choice.uuid
-    storage.set_study_system_attr(
-        study_id=study_id,
-        key=key,
-        value=json.dumps(choice, cls=Encoder),
-    )
diff --git a/python_tests/preferential/test_history.py b/python_tests/test_preferential_history.py
similarity index 100%
rename from python_tests/preferential/test_history.py
rename to python_tests/test_preferential_history.py