feat: support to cast struct to json (#2067)

Fixes internal issue 444196993

Author: chelsea-lin

bigframes/core/compile/ibis_compiler/scalar_op_registry.py

@@ -1023,6 +1023,8 @@ def astype_op_impl(x: ibis_types.Value, op: ops.AsTypeOp):
                 x, ibis_dtypes.string, safe=op.safe
             )
             return parse_json_in_safe(x_str) if op.safe else parse_json(x_str)
+        if x.type().is_struct():
+            return to_json_string(typing.cast(ibis_types.StructValue, x))
 
     if x.type() == ibis_dtypes.json:
         if to_type == ibis_dtypes.int64:
@@ -2069,7 +2071,7 @@ def json_extract_string_array(  # type: ignore[empty-body]
 
 @ibis_udf.scalar.builtin(name="to_json_string")
 def to_json_string(  # type: ignore[empty-body]
-    json_obj: ibis_dtypes.JSON,
+    json_obj,
 ) -> ibis_dtypes.String:
     """Convert JSON to STRING."""
 

bigframes/dtypes.py

@@ -641,6 +641,9 @@ def _dtype_from_string(dtype_string: str) -> typing.Optional[Dtype]:
         return BIGFRAMES_STRING_TO_BIGFRAMES[
             typing.cast(DtypeString, str(dtype_string))
         ]
+    if isinstance(dtype_string, str) and dtype_string.lower() == "json":
+        return JSON_DTYPE
+
     raise TypeError(
         textwrap.dedent(
             f"""
@@ -652,9 +655,9 @@ def _dtype_from_string(dtype_string: str) -> typing.Optional[Dtype]:
                         The following pandas.ExtensionDtype are supported:
                         pandas.BooleanDtype(), pandas.Float64Dtype(),
                         pandas.Int64Dtype(), pandas.StringDtype(storage="pyarrow"),
-                        pd.ArrowDtype(pa.date32()), pd.ArrowDtype(pa.time64("us")),
-                        pd.ArrowDtype(pa.timestamp("us")),
-                        pd.ArrowDtype(pa.timestamp("us", tz="UTC")).
+                        pandas.ArrowDtype(pa.date32()), pandas.ArrowDtype(pa.time64("us")),
+                        pandas.ArrowDtype(pa.timestamp("us")),
+                        pandas.ArrowDtype(pa.timestamp("us", tz="UTC")).
                 {constants.FEEDBACK_LINK}
                 """
         )

bigframes/operations/generic_ops.py

@@ -324,6 +324,8 @@ def _valid_cast(src: dtypes.Dtype, dst: dtypes.Dtype):
             if not _valid_cast(src_dtype, dst_dtype):
                 return False
         return True
+    if dtypes.is_struct_like(src) and dst == dtypes.JSON_DTYPE:
+        return True
 
     return _valid_scalar_cast(src, dst)
 

tests/system/small/test_series.py

@@ -3866,6 +3866,30 @@ def test_string_astype_timestamp():
     pd.testing.assert_series_equal(bf_result, pd_result, check_index_type=False)
 
 
+def test_struct_astype_json():
+    """See internal issue 444196993."""
+    s = series.Series(
+        [
+            {"version": 1, "project": "pandas"},
+            {"version": 2, "project": "numpy"},
+        ]
+    )
+    assert dtypes.is_struct_like(s.dtype)
+
+    expected = series.Series(s, dtype=dtypes.JSON_DTYPE)
+    assert expected.dtype == dtypes.JSON_DTYPE
+
+    result = s.astype("json")
+    pd.testing.assert_series_equal(
+        result.to_pandas(), expected.to_pandas(), check_index_type=False
+    )
+
+    result = s.astype(dtypes.JSON_DTYPE)
+    pd.testing.assert_series_equal(
+        result.to_pandas(), expected.to_pandas(), check_index_type=False
+    )
+
+
 def test_timestamp_astype_string():
     bf_series = series.Series(
         [