fix(models): allow nullable created_at/updated_at in BaseRecord

When records are deserialized from external sources (databases, APIs,
JSON responses), created_at and updated_at may legitimately be None.
Previously this raised a pydantic.ValidationError because the fields
were typed as datetime (non-nullable).

Change both fields in BaseRecord from datetime to datetime | None.
The default_factory still auto-populates with pendulum.now('UTC') for
freshly created objects, so existing code is unaffected.

This fixes the ValidationError reported in #8 where retrieving memory
categories with None timestamps caused 16 validation errors.

Closes #8

Author: sliverp

src/memu/database/models.py

@@ -36,8 +36,8 @@ class BaseRecord(BaseModel):
     """Backend-agnostic record interface."""
 
     id: str = Field(default_factory=lambda: str(uuid.uuid4()))
-    created_at: datetime = Field(default_factory=lambda: pendulum.now("UTC"))
-    updated_at: datetime = Field(default_factory=lambda: pendulum.now("UTC"))
+    created_at: datetime | None = Field(default_factory=lambda: pendulum.now("UTC"))
+    updated_at: datetime | None = Field(default_factory=lambda: pendulum.now("UTC"))
 
 
 class ToolCallResult(BaseModel):

tests/test_nullable_timestamps.py

@@ -0,0 +1,100 @@
+"""Unit tests for BaseRecord nullable datetime fields (issue #8 fix).
+
+When records are deserialized from external sources (databases, APIs, JSON),
+``created_at`` and ``updated_at`` may be ``None``.  Previously this raised a
+``pydantic.ValidationError`` because the fields were typed as ``datetime``
+(non-nullable).  After the fix they accept ``None`` gracefully while still
+auto-populating with ``pendulum.now("UTC")`` for freshly created objects.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+import pytest
+
+from memu.database.models import BaseRecord, CategoryItem, MemoryCategory, MemoryItem, Resource
+
+
+class TestBaseRecordNullableDatetime:
+    """Verify that ``created_at`` and ``updated_at`` accept ``None``."""
+
+    def test_default_factory_populates_datetime(self):
+        """Freshly created records should still get auto-populated timestamps."""
+        record = BaseRecord()
+        assert isinstance(record.created_at, datetime)
+        assert isinstance(record.updated_at, datetime)
+
+    def test_explicit_none_accepted(self):
+        """Passing ``None`` explicitly must not raise a ValidationError."""
+        record = BaseRecord(created_at=None, updated_at=None)
+        assert record.created_at is None
+        assert record.updated_at is None
+
+    def test_partial_none_accepted(self):
+        """One field ``None``, the other auto-populated — both valid."""
+        record = BaseRecord(created_at=None)
+        assert record.created_at is None
+        assert isinstance(record.updated_at, datetime)
+
+    def test_memory_item_with_none_timestamps(self):
+        """MemoryItem inherits BaseRecord; should tolerate ``None`` timestamps."""
+        item = MemoryItem(
+            resource_id=None,
+            memory_type="knowledge",
+            summary="test memory",
+            created_at=None,
+            updated_at=None,
+        )
+        assert item.created_at is None
+        assert item.updated_at is None
+
+    def test_memory_category_with_none_timestamps(self):
+        """MemoryCategory inherits BaseRecord; should tolerate ``None`` timestamps."""
+        cat = MemoryCategory(
+            name="test",
+            description="test category",
+            created_at=None,
+            updated_at=None,
+        )
+        assert cat.created_at is None
+
+    def test_resource_with_none_timestamps(self):
+        """Resource inherits BaseRecord; should tolerate ``None`` timestamps."""
+        res = Resource(
+            url="https://example.com",
+            modality="text",
+            local_path="/tmp/file.txt",
+            created_at=None,
+            updated_at=None,
+        )
+        assert res.created_at is None
+
+    def test_category_item_with_none_timestamps(self):
+        """CategoryItem inherits BaseRecord; should tolerate ``None`` timestamps."""
+        ci = CategoryItem(
+            item_id="item-1",
+            category_id="cat-1",
+            created_at=None,
+            updated_at=None,
+        )
+        assert ci.created_at is None
+
+    def test_model_dump_includes_none(self):
+        """Serialized output should faithfully represent ``None`` values."""
+        record = BaseRecord(created_at=None, updated_at=None)
+        data = record.model_dump()
+        assert data["created_at"] is None
+        assert data["updated_at"] is None
+
+    def test_model_validate_with_none(self):
+        """``model_validate`` from raw dict with ``None`` timestamps must work."""
+        data = {
+            "id": "abc-123",
+            "created_at": None,
+            "updated_at": None,
+        }
+        record = BaseRecord.model_validate(data)
+        assert record.id == "abc-123"
+        assert record.created_at is None
+        assert record.updated_at is None