id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36371	JSONField.from_db_value crashes when DB returns parsed JSON despite KeyTransform guard	Mason Pitts		"In Django 5.2, the default implementation of JSONField.from_db_value() only skips double-decoding when the ORM expression is a KeyTransform on a non-string. However, many modern database drivers (e.g. PostgreSQL psycopg3, Oracle DB_TYPE_JSON via cx_Oracle 8.1+/python-oracledb) will automatically deserialize JSON columns into native Python types (dict, list) before Django sees them. Since from_db_value() still unconditionally calls json.loads() in most cases, you get:


{{{
TypeError: the JSON object must be str, bytes or bytearray, not list
}}}

even though the value is already a valid Python object.

Here is the current code below as of 5/6/2025.


{{{
    def from_db_value(self, value, expression, connection):
        if value is None:
            return value
        # Some backends (SQLite at least) extract non-string values in their
        # SQL datatypes.
        if isinstance(expression, KeyTransform) and not isinstance(value, str):
            return value
        try:
            return json.loads(value, cls=self.decoder)
        except json.JSONDecodeError:
            return value
}}}


Here is a potential solution that attempts to return value if it is a Python type.


{{{
    def from_db_value(self, value, expression, connection):
        if value is None:
            return None

        # Decode binary data first.
        if isinstance(value, (bytes, bytearray)):
            value = value.decode()

        # If value isn’t a string at this point, the driver already gave us
        # a native Python type (dict, list, bool, int, float, ...).
        if not isinstance(value, str):
            return value

        try:
            return json.loads(value, cls=self.decoder)
        except json.JSONDecodeError:
            return value
}}}


Steps to reproduce:

1. Define a model with a models.JSONField().

2. Use a database and driver combination that natively decodes JSON columns—for example:

        - PostgreSQL with psycopg3

        - Oracle 21c+ JSON column type with cx_Oracle 8.1+ or python-oracledb in thin mode

* I encountered this problem using oracle_db in thin mode.

3. Query the model in the Django admin or via MyModel.objects.all().

4. Observe the traceback raising a TypeError when json.loads() is fed a list or dict.


Version information:

Django: 5.2

Python: 3.12.10

Affected drivers/backends:

PostgreSQL with psycopg3

Oracle 21c+ with cx_Oracle 8.1+ / python-oracledb in thin mode
"	Bug	closed	Database layer (models, ORM)	5.2	Normal	worksforme	jsonfield, from_db_value, double-decoding, psycopg3, cx_oracle, python-oracledb	Mason Pitts	Unreviewed	0	0	0	0	0	0