fix: Deserializing JSON subfields within structs fails (#1742)

… for deserializing json subfields from bigquery, this adds support for that.

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- [x] Make sure to open an issue as a [bug/issue](https://togithub.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

Fixes #[1500](https://togithub.com/googleapis/python-bigquery/issues/1500) 🦕

google/cloud/bigquery/_helpers.py

@@ -239,6 +239,15 @@ def _record_from_json(value, field):
         return record
 
 
+def _json_from_json(value, field):
+    """Coerce 'value' to a Pythonic JSON representation."""
+    if _not_null(value, field):
+        return json.loads(value)
+    else:
+        return None
+
+
+# Parse BigQuery API response JSON into a Python representation.
 _CELLDATA_FROM_JSON = {
     "INTEGER": _int_from_json,
     "INT64": _int_from_json,
@@ -257,6 +266,7 @@ def _record_from_json(value, field):
     "DATE": _date_from_json,
     "TIME": _time_from_json,
     "RECORD": _record_from_json,
+    "JSON": _json_from_json,
 }
 
 _QUERY_PARAMS_FROM_JSON = dict(_CELLDATA_FROM_JSON)
@@ -413,13 +423,8 @@ def _time_to_json(value):
     return value
 
 
-def _json_from_json(value, field):
-    """Coerce 'value' to a pythonic JSON representation, if set or not nullable."""
-    if _not_null(value, field):
-        return json.loads(value)
-
-
-# Converters used for scalar values marshalled as row data.
+# Converters used for scalar values marshalled to the BigQuery API, such as in
+# query parameters or the tabledata.insert API.
 _SCALAR_VALUE_TO_JSON_ROW = {
     "INTEGER": _int_to_json,
     "INT64": _int_to_json,

tests/unit/test__helpers.py

@@ -15,6 +15,7 @@
 import base64
 import datetime
 import decimal
+import json
 import unittest
 
 import mock
@@ -71,9 +72,20 @@ def test_w_none_required(self):
         with self.assertRaises(TypeError):
             self._call_fut(None, _Field("REQUIRED"))
 
+    def test_w_json_field(self):
+        data_field = _Field("REQUIRED", "data", "JSON")
+
+        value = json.dumps(
+            {"v": {"key": "value"}},
+        )
+
+        expected_output = {"v": {"key": "value"}}
+        coerced_output = self._call_fut(value, data_field)
+        self.assertEqual(coerced_output, expected_output)
+
     def test_w_string_value(self):
-        coerced = self._call_fut('{"foo": true}', object())
-        self.assertEqual(coerced, {"foo": True})
+        coerced = self._call_fut('"foo"', object())
+        self.assertEqual(coerced, "foo")
 
 
 class Test_float_from_json(unittest.TestCase):