fix: failure Loading json columns into an arrow table

dlt/common/libs/pyarrow.py

@@ -4,7 +4,6 @@
     Any,
     Dict,
     Mapping,
-    Tuple,
     Optional,
     Union,
     Callable,
@@ -670,9 +669,16 @@ def row_tuples_to_arrow(
 
     arrow_schema = columns_to_arrow(columns, caps, tz)
 
+    def infer_first_non_null_type(idx: int) -> Any:
+        for row in rows:
+            value = row[idx]
+            if value is not None:
+                return type(value)
+        return type(None)
+
     for idx in range(0, len(arrow_schema.names)):
         field = arrow_schema.field(idx)
-        py_type = type(rows[0][idx])
+        py_type = infer_first_non_null_type(idx)
         # cast double / float ndarrays to decimals if type mismatch, looks like decimals and floats are often mixed up in dialects
         if pa.types.is_decimal(field.type) and issubclass(py_type, (str, float)):
             logger.warning(

tests/sources/sql_database/test_arrow_helpers.py

@@ -83,6 +83,39 @@ def test_row_tuples_to_arrow_unknown_types(all_unknown: bool) -> None:
     assert pa.types.is_list(result[7].type)
 
 
+@pytest.mark.parametrize("all_unknown", [True, False])
+def test_row_tuples_to_arrow_detects_nullable_json(all_unknown: bool) -> None:
+    rows = [
+        (1, None),
+        (2, {"ix": 2}),
+    ]
+
+    columns = {
+        "int_col": {"name": "int_col", "data_type": "bigint", "nullable": False},
+        "json_col": {"name": "json_col", "data_type": "json", "nullable": False},
+    }
+
+    if all_unknown:
+        for col in columns.values():
+            col.pop("data_type", None)
+
+    result = row_tuples_to_arrow(rows, columns=columns, tz="UTC")  # type: ignore
+
+    # Result is arrow table containing all columns in original order with correct types
+    assert result.num_columns == len(columns)
+    result_col_names = [f.name for f in result.schema]
+    expected_names = list(columns)
+    assert result_col_names == expected_names
+
+    assert pa.types.is_int64(result[0].type)
+
+    # FIXME Why isn't this always coerced to a string?
+    if all_unknown:
+        assert pa.types.is_struct(result[1].type)
+    else:
+        assert pa.types.is_string(result[1].type)
+
+
 pytest.importorskip("sqlalchemy", minversion="2.0")