fix json nullable column

ormar/fields/model_fields.py

@@ -168,7 +168,7 @@ class ModelFieldFactory:
             unique=kwargs.pop("unique", False),
             pydantic_only=pydantic_only,
             autoincrement=autoincrement,
-            column_type=cls.get_column_type(**kwargs),
+            column_type=cls.get_column_type(**kwargs, sql_nullable=sql_nullable),
             choices=choices,
             encrypt_secret=encrypt_secret,
             encrypt_backend=encrypt_backend,
@@ -516,7 +516,7 @@ class JSON(ModelFieldFactory, pydantic.Json):
         :return: initialized column with proper options
         :rtype: sqlalchemy Column
         """
-        return sqlalchemy.JSON()
+        return sqlalchemy.JSON(none_as_null=kwargs.get("sql_nullable", False))
 
 
 if TYPE_CHECKING:  # pragma: nocover # noqa: C901

ormar/fields/parsers.py

@@ -2,7 +2,7 @@ import base64
 import datetime
 import decimal
 import uuid
-from typing import Any, Callable, Dict, Union
+from typing import Any, Callable, Dict, Optional, Union
 
 import pydantic
 from pydantic.datetime_parse import parse_date, parse_datetime, parse_time
@@ -37,7 +37,7 @@ def encode_bytes(value: Union[str, bytes], represent_as_string: bool = False) ->
     return value if isinstance(value, bytes) else value.encode("utf-8")
 
 
-def encode_json(value: Any) -> str:
+def encode_json(value: Any) -> Optional[str]:
     value = json.dumps(value) if not isinstance(value, str) else re_dump_value(value)
     value = value.decode("utf-8") if isinstance(value, bytes) else value
     return value

ormar/models/newbasemodel.py

@@ -22,14 +22,11 @@ from typing import (
 import databases
 import pydantic
 import sqlalchemy
+
+from ormar.fields.parsers import encode_json
 from ormar.models.utils import Extra
 from pydantic import BaseModel
 
-try:
-    import orjson as json
-except ImportError:  # pragma: no cover
-    import json  # type: ignore
-
 
 import ormar  # noqa I100
 from ormar.exceptions import ModelError, ModelPersistenceError
@@ -925,12 +922,7 @@ class NewBaseModel(pydantic.BaseModel, ModelTableProxy, metaclass=ModelMetaclass
         """
         if column_name not in self._json_fields:
             return value
-        if not isinstance(value, str):
-            try:
-                value = json.dumps(value)
-            except TypeError:  # pragma no cover
-                pass
-        return value.decode("utf-8") if isinstance(value, bytes) else value
+        return encode_json(value)
 
     def _extract_own_model_fields(self) -> Dict:
         """