Resolve lazy objects inside dictionaries when saving JSONFields

[Jacob Walls]

I have a model Notification like this:

class Notification(models.Model):
    message = models.TextField(null=True)
    context = models.JSONField(null=True)

Here is the lazy object...

>>> from django.utils.translation import gettext_lazy as _
>>> lazy = _("lazy")

... that I can use in a TextField ...

>>> models.Notification(message=lazy).save()
>>> 

... but not for the JSONField without string-ifying it myself. Suggesting an implementation that resolves these in one of the methods that prepares values for database insertion. Tested with psycopg2 2.9.9.

>>> models.Notification(context={'lazy': lazy}).save()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/base.py", line 814, in save
    self.save_base(
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/base.py", line 877, in save_base
    updated = self._save_table(
              ^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/base.py", line 990, in _save_table
    updated = self._do_update(
              ^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/base.py", line 1054, in _do_update
    return filtered._update(values) > 0
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/query.py", line 1231, in _update
    return query.get_compiler(self.db).execute_sql(CURSOR)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 1982, in execute_sql
    cursor = super().execute_sql(result_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 1560, in execute_sql
    cursor.execute(sql, params)
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/backends/utils.py", line 102, in execute
    return super().execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/backends/utils.py", line 67, in execute
    return self._execute_with_wrappers(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/backends/utils.py", line 80, in _execute_with_wrappers
    return executor(sql, params, many, context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/django/db/backends/utils.py", line 89, in _execute
    return self.cursor.execute(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/psycopg2/_json.py", line 78, in getquoted
    s = self.dumps(self.adapted)
        ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jwalls/venv311/lib/python3.11/site-packages/psycopg2/_json.py", line 72, in dumps
    return self._dumps(obj)
           ^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 180, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type __proxy__ is not JSON serializable

[Mariusz Felisiak]

It's easier to resolve a type for string-based fields, however, JSONField's values can contain different types of data. I don't think it's worth adding extra guess-like complexity for this. We can reconsider this decision if you provide a PoC.

[Jacob Walls]

I was thinking of something like this:

django/db/models/fields/json.py

@@ +1 @@
+from collections.abc import Container
 import json
 
 from django import forms
+from django.conf import settings
 from django.core import checks, exceptions
 from django.db import NotSupportedError, connections, router
 from django.db.models import expressions, lookups
@@ -11 +13 @@
     PostgresOperatorLookup,
     Transform,
 )
+from django.utils.functional import Promise
 from django.utils.translation import gettext_lazy as _
 
 from . import Field
@@ -96 +99 @@
     def get_internal_type(self):
         return "JSONField"
 
+    def get_prep_value(self, value):
+        if settings.USE_I18N:
+            match value:
+                case dict():
+                    value = {
+                        self.get_prep_value(k): self.get_prep_value(v)
+                        for k, v in value.items()
+                    }
+                case str() | Promise():
+                    pass
+                case Container():
+                    value = value.__class__(self.get_prep_value(v) for v in value)
+
+        return super().get_prep_value(value)
+
     def get_db_prep_value(self, value, connection, prepared=False):
         if not prepared:
             value = self.get_prep_value(value)

[Mariusz Felisiak]

Why do you want to put it behind the USE_I18N?

[Jacob Walls]

Yeah, I hadn't yet looked into whether lazy translation objects get resolved when USE_I18N = False. Looks like they do, so I should remove the guard.

[Mariusz Felisiak]

Tentatively accepted.

[Jacob Walls]

So I have something sort of ready to go here, but then I noticed that what I should probably be doing is using DjangoJSONEncoder in the encoder arg of JSONField, which solves my use case. Is that enough? Should that be the default?

[Jacob Walls]

I guess it would still be nice to separate this concern (resolving the promise) from the choice of serializer.

[Jacob Walls]

​PR

[Claude Paroz]

I think using the encoder arg of JSONField is the right answer to this issue. The get_prep_value of your suggested patch would add too much overhead in my opinion.

[Simon Charette]

I completely agree with Claude here.

As a matter of fact DjangoJSONEncoder already works perfectly for this use case so I don't understand why we should treat promises differently ​given we already document using it for such cases and that the latter is ​documented to work with promises.

import json
from django.core.serializers.json import DjangoJSONEncoder
from django.utils.translation import gettext_lazy as _

json.dumps({"key": _("foobar")})
# TypeError: Object of type __proxy__ is not JSON serializable
json.dumps({"key": _("foobar")}, cls=DjangoJSONEncoder)
>>> '{"key": "foobar"}'

We also can't make encoder default to DjangoJSONEncoder as it elides the original type so you'd be able to serialize uuid, datetimes, etc but not get them back which breaks the principle of least astonishment IMO. DjangoJSONEncoder works well for the serialization framework as it is equipped which a schema (model definition) so type information can be restored but JSONField are schema less so it's not a good default.

[Jacob Walls]

Makes sense. Docs links are very helpful. Sorry for not using a support channel to check my understanding first!