Fix nullable type handling for column types

target_clickhouse/connectors.py

@@ -128,6 +128,17 @@ def to_sql_type(
         ):
             sql_type = clickhouse_sqlalchemy_types.Nullable(sql_type)
 
+        # Wrap any type in Nullable if the JSON schema allows null values
+        # and it's not already Nullable and not a primary key.
+        schema_type = jsonschema_type.get("type", [])
+        if (
+            isinstance(schema_type, list)
+            and "null" in schema_type
+            and not is_primary_key
+            and not isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+        ):
+            sql_type = clickhouse_sqlalchemy_types.Nullable(sql_type)
+
         return sql_type
 
     def create_empty_table(

tests/test_nullable_types.py

@@ -0,0 +1,68 @@
+"""Tests for nullable type handling in to_sql_type."""
+
+from clickhouse_sqlalchemy import types as clickhouse_sqlalchemy_types
+
+from target_clickhouse.connectors import ClickhouseConnector
+
+
+def _to_sql_type(jsonschema_type, is_primary_key=False):
+    """Call to_sql_type as an unbound method — avoids full connector init."""
+    return ClickhouseConnector.to_sql_type(
+        None,
+        jsonschema_type,
+        is_primary_key=is_primary_key,
+    )
+
+
+class TestNullableTypeMapping:
+    """Tests for nullable type wrapping in ClickhouseConnector.to_sql_type."""
+
+    def test_nullable_bool_returns_nullable(self):
+        """Nullable bool schema should produce Nullable(Bool)."""
+        sql_type = _to_sql_type({"type": ["boolean", "null"]})
+        assert isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_non_nullable_bool_returns_plain_bool(self):
+        """Non-nullable bool schema should not be wrapped."""
+        sql_type = _to_sql_type({"type": ["boolean"]})
+        assert not isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_nullable_integer_returns_nullable(self):
+        """Nullable integer schema should produce Nullable(Int64)."""
+        sql_type = _to_sql_type({"type": ["integer", "null"]})
+        assert isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_nullable_string_returns_nullable(self):
+        """Nullable string schema should produce Nullable(String)."""
+        sql_type = _to_sql_type({"type": ["string", "null"]})
+        assert isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_nullable_number_returns_nullable(self):
+        """Nullable number schema should produce Nullable(Float)."""
+        sql_type = _to_sql_type({"type": ["number", "null"]})
+        assert isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_nullable_datetime_stays_nullable(self):
+        """Datetime was already wrapped — should not double-wrap."""
+        sql_type = _to_sql_type(
+            {"type": ["string", "null"], "format": "date-time"},
+        )
+        assert isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_primary_key_not_nullable(self):
+        """Primary key columns should never be Nullable."""
+        sql_type = _to_sql_type(
+            {"type": ["boolean", "null"]},
+            is_primary_key=True,
+        )
+        assert not isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)
+
+    def test_non_nullable_integer_returns_int64(self):
+        """Non-nullable integer should produce Int64."""
+        sql_type = _to_sql_type({"type": ["integer"]})
+        assert isinstance(sql_type, clickhouse_sqlalchemy_types.Int64)
+
+    def test_string_type_not_list(self):
+        """Handle case where type is a string, not a list."""
+        sql_type = _to_sql_type({"type": "boolean"})
+        assert not isinstance(sql_type, clickhouse_sqlalchemy_types.Nullable)